Filed Pursuant to Rule 424(b)(3)
Registration No. 333-229931

MERGER PROPOSAL—YOUR VOTE IS VERY IMPORTANT

Dear Stockholder:

On January 27, 2019, Entegris, Inc., which is referred to as Entegris, and Versum Materials, Inc., which is referred to as Versum, entered into an Agreement and Plan of Merger, as it may be amended from time to time, which is referred to as the merger agreement, pursuant to which they agreed to combine their respective businesses in a merger of equals. The combined company will be a premier specialty materials company for the semiconductor and other high-tech industries. Pursuant to the terms of the merger agreement, Versum will merge with and into Entegris, which transaction is referred to as the merger, with Entegris as the surviving corporation. Following the merger, the combined company will retain the Entegris name.

Upon successful completion of the merger, each issued and outstanding share of Versum common stock will be converted into the right to receive 1.120 shares of Entegris common stock, which number is referred to as the exchange ratio. This exchange ratio is fixed and will not be adjusted for changes in the market price of either Entegris common stock or Versum common stock between the dates of signing of the merger agreement and completion of the merger. Entegris stockholders will continue to own their existing Entegris shares. Based on the fully-vested shares outstanding of each of Entegris and Versum, including exercisable options only, Entegris stockholders will own approximately 52.5% and Versum stockholders will own approximately 47.5% of the issued and outstanding shares of the combined company (based on fully diluted shares outstanding of the combined company including exercisable options only) immediately following the completion of the merger. Entegris common stock is traded on the NASDAQ Global Select Market under the symbol “ENTG.” Versum common stock is traded on the NYSE under the symbol “VSM.” The common stock of the combined company is expected to be listed on NASDAQ or the NYSE under the symbol “ENTG.” Stockholders will be informed in conjunction with the closing on which exchange the shares will trade. We encourage you to obtain updated quotes for the common stock of both Entegris and Versum.

Entegris and Versum will each hold special meetings of their respective stockholders in connection with the proposed merger, which are referred to as the Entegris special meeting and the Versum special meeting, respectively.

At the Entegris special meeting, Entegris stockholders will be asked to consider and vote on (1) the proposal to adopt the merger agreement (including the issuance of shares of Entegris common stock to Versum stockholders pursuant to the merger agreement), which proposal is referred to as the Entegris merger agreement proposal, (2) the proposal to adopt the amended and restated certificate of incorporation of Entegris, (3) the proposal to approve, on a non-binding advisory basis, specific compensatory arrangements between Entegris and its named executive officers relating to the merger and (4) the proposal to adjourn the Entegris special meeting to solicit additional proxies if there are not sufficient votes to approve the Entegris merger agreement proposal or the Entegris charter proposal or to ensure that any supplement or amendment to the accompanying joint proxy statement/prospectus is timely provided to Entegris stockholders. The board of directors of Entegris unanimously recommends that Entegris stockholders vote “FOR” each of the proposals to be considered at the Entegris special meeting.

At the Versum special meeting, Versum stockholders will be asked to consider and vote on (1) the proposal to adopt the merger agreement, which is referred to as the Versum merger agreement proposal, (2) the proposal to approve, on a non-binding advisory basis, specific compensatory arrangements between Versum and its named executive officers relating to the merger and (3) the proposal to adjourn the Versum special meeting to solicit additional proxies if there are not sufficient votes to approve the Versum merger agreement proposal or to ensure that any supplement or amendment to the accompanying joint proxy statement/prospectus is timely provided to Versum stockholders. The board of directors of Versum unanimously recommends that Versum stockholders vote “FOR” each of the proposals to be considered at the Versum special meeting.

We cannot complete the merger unless Entegris stockholders approve the Entegris merger agreement proposal and Versum stockholders approve the Versum merger agreement proposal. Your vote on these matters is very important, regardless of the number of shares you own. Whether or not you plan to attend your respective special meeting in person, please promptly mark, sign and date the accompanying proxy and return it in the enclosed postage-paid envelope or authorize the individuals named on your WHITE proxy card to vote your shares by calling the toll-free telephone number or by using the Internet as described in the instructions included with your WHITE proxy card.

The accompanying joint proxy statement/prospectus provides you with important information about the special meetings, the merger, and each of the proposals. We encourage you to read the entire document carefully, in particular the “Risk Factors” section beginning on page 43 for a discussion of risks relevant to the merger.

We look forward to the successful completion of the merger.

Sincerely,

Neither the U.S. Securities and Exchange Commission nor any state securities commission has approved or disapproved of the merger or the Entegris common stock to be issued in the merger or determined if this joint proxy statement/prospectus is accurate or complete. Any representation to the contrary is a criminal offense.

This joint proxy statement/prospectus is dated March 20, 2019 and is first being mailed to the stockholders of Entegris and Versum with the WHITE proxy card on or about March 22, 2019.

Entegris, Inc.

129 Concord Road

Billerica, Massachusetts 01821

(978) 436-6500

NOTICE OF SPECIAL MEETING OF STOCKHOLDERS

TO BE HELD ON APRIL 26, 2019

To the Stockholders of Entegris, Inc.:

Notice is hereby given that Entegris, Inc., which is referred to as Entegris, will hold a special meeting of its stockholders, which is referred to as the Entegris special meeting, at Entegris’ headquarters at 129 Concord Road, Billerica, Massachusetts, on April 26, 2019, beginning at 12:00 p.m., Eastern Time, for the purpose of considering and voting on the following proposals:

Entegris will transact no other business at the Entegris special meeting except such business as may properly be brought before the Entegris special meeting or any adjournment or postponement thereof. The accompanying joint proxy statement/prospectus, including the merger agreement attached thereto as Annex A, contains further information with respect to these matters.

Only holders of record of Entegris common stock at the close of business on April 2, 2019, the record date for voting at the Entegris special meeting, which is referred to as the Entegris record date, are entitled to notice of and to vote at the Entegris special meeting and any adjournments or postponements thereof.

The board of directors of Entegris, which is referred to as the Entegris board of directors, has unanimously approved and declared advisable the merger agreement and the transactions contemplated by the merger agreement on the terms and subject to the conditions set forth in the merger agreement. The Entegris board of directors unanimously recommends that Entegris stockholders vote “FOR” the Entegris merger agreement proposal, “FOR” the Entegris charter proposal, “FOR” the Entegris compensation proposal and “FOR” the Entegris adjournment proposal.

Your vote is very important, regardless of the number of shares of Entegris common stock you own. Entegris cannot complete the transactions contemplated by the merger agreement without approval of the Entegris merger agreement proposal. Assuming a quorum is present, the approval of the Entegris merger

agreement proposal requires the affirmative vote of a majority of the outstanding shares of Entegris common stock entitled to vote on the proposal.

Whether or not you plan to attend the Entegris special meeting in person, Entegris urges you to please promptly mark, sign and date the accompanying proxy and return it in the enclosed postage-paid envelope, call the toll-free telephone number or use the Internet as described in the instructions included with the WHITE proxy card, so that your shares may be represented and voted at the Entegris special meeting. If you hold your shares through a broker, bank or other nominee in “street name” (instead of as a registered holder) and you wish to vote in person at the Entegris special meeting, you must obtain a legal proxy from your bank, broker or other nominee and bring the legal proxy to the meeting in order to vote in person at the Entegris special meeting. You will need to bring identification along with either your notice of special meeting or proof of stock ownership to enter the Entegris special meeting. The use of video, still photography or audio recording at the Entegris special meeting is not permitted. For the safety of attendees, all bags, packages and briefcases are subject to inspection.

If you have any questions about the merger, please contact Entegris at (978) 436-6500 or write to Entegris, Inc., Attn: Corporate Secretary, 129 Concord Road, Billerica, Massachusetts 01821.

If you have any questions about how to vote or direct a vote in respect of your shares of Entegris common stock, you may contact our proxy solicitor, MacKenzie Partners, Inc., at (800) 322-2885.

By Order of the Board of Directors,

Bertrand Loy

President and Chief Executive Officer

Billerica, Massachusetts

Dated: March 20, 2019

Versum Materials, Inc.

8555 South River Parkway

Tempe, Arizona 85284

(602) 282-1000

NOTICE OF SPECIAL MEETING OF STOCKHOLDERS

TO BE HELD ON APRIL 26, 2019

To the Stockholders of Versum Materials, Inc.:

Notice is hereby given that Versum Materials, Inc., which is referred to as Versum, will hold a special meeting of its stockholders, which is referred to as the Versum special meeting, at 8555 South River Parkway, Tempe, Arizona 85284, on April 26, 2019, beginning at 9:00 a.m., Mountain Standard Time, for the purpose of considering and voting on the following proposals:

Versum will transact no other business at the Versum special meeting except such business as may properly be brought before the Versum special meeting or any adjournment or postponement thereof. The accompanying joint proxy statement/prospectus, including the merger agreement attached thereto as Annex A, contains further information with respect to these matters.

Only holders of record of Versum common stock at the close of business on April 2, 2019, the record date for voting at the Versum special meeting, which is referred to as the Versum record date, are entitled to notice of and to vote at the Versum special meeting and any adjournments or postponements thereof.

The board of directors of Versum, which is referred to as the Versum board of directors, has unanimously approved and declared advisable the merger agreement and the transactions contemplated by the merger agreement, including the merger, on the terms and subject to the conditions set forth in the merger agreement. The Versum board of directors unanimously recommends that Versum stockholders vote “FOR” the Versum merger agreement proposal, “FOR” the Versum compensation proposal and “FOR” the Versum adjournment proposal.

Your vote is very important, regardless of the number of shares of Versum common stock you own. Versum cannot complete the transactions contemplated by the merger agreement, including the merger, without approval of the Versum merger agreement proposal. Assuming a quorum is present, the approval of the Versum merger agreement proposal requires the affirmative vote of a majority of the outstanding shares of Versum common stock entitled to vote on the Versum merger agreement proposal.

Whether or not you plan to attend the Versum special meeting in person, Versum urges you to please promptly mark, sign and date the accompanying proxy and return it in the enclosed postage-paid envelope, call the toll-free telephone number or use the Internet as described in the instructions included with the WHITE proxy card, so that your shares may be represented and voted at the Versum special meeting. If you hold your shares through a broker, bank or other nominee in “street name” (instead of as a registered holder) and you wish to vote in person at the Versum special meeting, you must obtain a legal proxy from your bank, broker or other nominee and bring the legal proxy to the meeting in order to vote in person at the Versum special meeting. You will need to bring identification along with either your notice of special meeting or proof of stock ownership to enter the Versum special meeting. The use of video, still photography or audio recording at the Versum special meeting is not permitted. For the safety of attendees, all bags, packages and briefcases are subject to inspection.

We urge you to discard any green proxy cards, which may have been sent to you by Merck KGaA, Darmstadt, Germany, who is soliciting proxies from Versum stockholders in opposition to the merger. If you previously submitted a green proxy card, we urge you to cast your vote as instructed on your WHITE proxy card, which will revoke any earlier dated proxy card that you may have submitted, including any green proxy card. Only the latest validly executed proxy that you submit will be counted.

If you have any questions about the merger, please contact Versum at (602) 282-1000 or write to Versum Materials, Inc., Attn: Corporate Secretary, 8555 South River Parkway, Tempe, Arizona 85284.

If you have any questions about how to vote or direct a vote in respect of your shares of Versum common stock, you may contact Versum’s proxy solicitor, Innisfree M&A Incorporated, toll-free at (877) 456-3463 or call collect at (212) 750-5833.

By Order of the Board of Directors,

Michael W. Valente

Senior Vice President, General Counsel and Secretary

Tempe, Arizona

Dated: March 20, 2019

REFERENCES TO ADDITIONAL INFORMATION

This joint proxy statement/prospectus incorporates important business and financial information about Entegris and Versum from other documents that Entegris and Versum have filed with the U.S. Securities and Exchange Commission, which is referred to as the SEC, and that are contained in or incorporated by reference into this joint proxy statement/prospectus. For a listing of documents incorporated by reference into this joint proxy statement/prospectus, please see the section entitled “Where You Can Find More Information” beginning on page 213. This information is available for you free of charge to review through the SEC’s website at www.sec.gov.

Any person may request a copy of this joint proxy statement/prospectus and any of the documents incorporated by reference into this joint proxy statement/prospectus or other information concerning Entegris or Versum, without charge, by written or telephonic request directed to the appropriate company or its proxy solicitor at the following contacts:

In order for you to receive timely delivery of the documents in advance of the special meeting of Entegris stockholders to be held on April 26, 2019, which is referred to as the Entegris special meeting, or the special meeting of Versum stockholders to be held on April 26, 2019, which is referred to as the Versum special meeting, as applicable, you must request the information no later than April 19, 2019.

The contents of the websites of the SEC, Entegris, Versum or any other entity are not being incorporated into this joint proxy statement/prospectus. The information about how you can obtain certain documents that are incorporated by reference into this joint proxy statement/prospectus at these websites is being provided only for your convenience.

ABOUT THIS JOINT PROXY STATEMENT/PROSPECTUS

This document, which forms part of a registration statement on Form S-4 filed with the SEC by Entegris, constitutes a prospectus of Entegris under Section 5 of the Securities Act of 1933, as amended, which is referred to as the Securities Act, with respect to the shares of common stock of Entegris to be issued to Versum stockholders pursuant to the Agreement and Plan of Merger, dated as of January 27, 2019, by and between Entegris and Versum, as it may be amended from time to time, which is referred to as the merger agreement. This document also constitutes a joint proxy statement of Entegris and Versum under Section 14(a) of the Securities Exchange Act of 1934, as amended, which is referred to as the Exchange Act. It also constitutes a notice of meeting with respect to the Entegris special meeting and a notice of meeting with respect to the Versum special meeting.

Entegris has supplied all information contained or incorporated by reference into this joint proxy statement/prospectus relating to Entegris, and Versum has supplied all such information relating to Versum. Entegris and Versum have both contributed to the information related to the merger contained in this joint proxy statement/prospectus.

You should rely only on the information contained in or incorporated by reference into this joint proxy statement/prospectus. Entegris and Versum have not authorized anyone to provide you with information that is different from that contained in or incorporated by reference into this joint proxy statement/prospectus. This joint proxy statement/prospectus is dated March 20, 2019, and you should not assume that the information contained in this joint proxy statement/prospectus is accurate as of any date other than such date unless otherwise specifically provided herein.

Further, you should not assume that the information incorporated by reference into this joint proxy statement/prospectus is accurate as of any date other than the date of the incorporated document. Neither the mailing of this joint proxy statement/prospectus to Entegris stockholders or Versum stockholders nor the issuance by Entegris of shares of its common stock pursuant to the merger agreement will create any implication to the contrary.

This joint proxy statement/prospectus does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction to or from any person to whom it is unlawful to make any such offer or solicitation in such jurisdiction.

All references in this joint proxy statement/prospectus to “Entegris” refer to Entegris, Inc., a Delaware corporation. All references in this joint proxy statement/prospectus to “Versum” refer to Versum Materials, Inc., a Delaware corporation. All references in this joint proxy statement/prospectus to the “combined company” refer to Entegris immediately following completion of the merger and the other transactions contemplated by the merger agreement. All references in this joint proxy statement/prospectus to “Entegris common stock” refer to the common stock of Entegris, par value $0.01 per share, and all references in this joint proxy statement/prospectus to “Versum common stock” refer to the common stock of Versum, par value $1.00 per share.

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QUESTIONS AND ANSWERS

The following are some questions that you, as a stockholder of Entegris or a stockholder of Versum, may have regarding the merger and the other matters being considered at the special meetings of each company’s stockholders, and brief answers to those questions. You are urged to carefully read this joint proxy statement/prospectus and the other documents referred to in this joint proxy statement/prospectus in their entirety because this section may not provide all the information that is important to you regarding these matters. Additional important information is contained in the annexes to, and the documents incorporated by reference into, this joint proxy statement/prospectus. You may obtain the information incorporated by reference in this joint proxy statement/prospectus, without charge, by following the instructions under the section entitled “Where You Can Find More Information” beginning on page 213.

In order to complete the merger, among other things:

Entegris is holding a special meeting of its stockholders, which is referred to as the Entegris special meeting, to obtain approval of the Entegris merger agreement proposal. Entegris stockholders will also be asked to adopt Entegris’ amended and restated certificate of incorporation, which proposal is referred to as the Entegris charter proposal, to approve, on an advisory (non-binding) basis, the merger-related executive officer compensation payments that will or may be paid by Entegris to its named executive officers in connection with the merger, which proposal is referred to as the Entegris compensation proposal, and to approve the proposal to adjourn the Entegris special meeting to solicit additional proxies if there are not sufficient votes at the time of the Entegris special meeting to approve the Entegris merger agreement proposal and the Entegris charter proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to Entegris stockholders, which proposal is referred to as the Entegris adjournment proposal.

Versum is holding a special meeting of its stockholders, which is referred to as the Versum special meeting, to obtain approval of the Versum merger agreement proposal. Versum stockholders will also be asked to approve, on an advisory (non-binding) basis, the merger-related executive officer compensation payments that will or may be paid by Versum to its named executive officers in connection with the merger, which proposal is referred to as the Versum compensation proposal, and to approve the proposal to adjourn the Versum special meeting to solicit additional proxies if there are not sufficient votes at the time of the Versum special meeting to approve the Versum merger agreement proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to Versum stockholders, which proposal is referred to as the Versum adjournment proposal.

Your vote is very important, regardless of the number of shares that you own. The approval of the Entegris merger agreement proposal and the approval of the Versum merger agreement proposal are conditions to the

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obligations of Versum and Entegris to complete the merger. None of the approvals of the Entegris charter proposal, the Entegris compensation proposal, the Versum compensation proposal, the Entegris adjournment proposal or the Versum adjournment proposal are conditions to the obligations of Versum or Entegris to complete the merger.

The Versum special meeting will be held at 8555 South River Parkway, Tempe, Arizona 85284, on April 26, 2019, at 9:00 a.m., Mountain Standard Time.

If you choose to vote your shares in person at your respective company’s special meeting, please bring required documentation in accordance with the section entitled “The Entegris Special Meeting—Attending the Entegris Special Meeting” beginning on page 63, with respect to the Entegris special meeting, and the section entitled “The Versum Special Meeting—Attending the Versum Special Meeting” beginning on page 74, with respect to the Versum special meeting. The use of video, still photography or audio recording at each of the special meetings is not permitted. For the safety of attendees, all bags, packages and briefcases are subject to inspection.

Even if you plan to attend your respective company’s special meeting, Versum and Entegris recommend that you vote your shares in advance as described below so that your vote will be counted if you later decide not to or become unable to attend the applicable special meeting. Shares held in “street name” may be voted in person by you only if you obtain a signed legal proxy from your bank, broker or other nominee giving you the right to vote the shares.

For Entegris stockholders, if you do not return or submit your proxy or vote at the special meeting as provided in this joint proxy statement/prospectus, the effect will be the same as a vote “AGAINST” the Entegris merger agreement proposal and the Entegris charter proposal, and will have no effect on the Entegris compensation proposal or the Entegris adjournment proposal. The board of directors of Entegris, which is referred to as the Entegris board of directors, unanimously recommends that you vote “FOR” the Entegris merger agreement proposal, “FOR” the Entegris charter proposal, “FOR” the Entegris compensation proposal and “FOR” the Entegris adjournment proposal.

For Versum stockholders, if you do not return or submit your proxy or vote at the special meeting as provided in this joint proxy statement/prospectus, the effect will be the same as a vote “AGAINST” the Versum merger agreement proposal, and will have no effect on the Versum compensation proposal or the Versum adjournment proposal. The Versum board of directors, which is referred to as the Versum board of directors, unanimously recommends that you vote “FOR” the Versum merger agreement proposal, “FOR” the Versum compensation proposal and “FOR” the Versum adjournment proposal.

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If the merger is completed, Entegris stockholders’ shares of Entegris common stock will, after the effective time, constitute shares of the combined company.

Because Entegris will issue a fixed number of shares of Entegris common stock in exchange for each share of Versum common stock, the value of the merger consideration that Versum stockholders will receive in the merger will depend on the market price of shares of Entegris common stock at the time the merger is completed. The market price of shares of Entegris common stock that Versum stockholders receive after the merger is completed could be greater than, less than or the same as the market price of shares of Entegris common stock on the date of this joint proxy statement/prospectus or at the time of the special meetings. Accordingly, you should obtain current market quotations for Entegris common stock and Versum common stock before deciding how to vote with respect to the adoption of the merger agreement. Entegris common stock is traded on the NASDAQ Global Select market, which is referred to as NASDAQ, and Versum common stock is traded on the New York Stock Exchange, which is referred to as the NYSE, under the symbols “ENTG” and “VSM,” respectively. Shares of common stock of the combined company are expected to trade on NASDAQ or the NYSE under the symbol “ENTG” after completion of the merger. Stockholders will be informed in conjunction with the closing on which exchange the shares will trade.

For more information regarding the merger consideration to be received by Versum stockholders if the merger is completed, see the section entitled “The Merger Agreement—Merger Consideration” beginning on page 128.

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At the effective time, each Versum time-based restricted stock unit award (including equity-based awards granted by Air Products and Chemicals, Inc. and its consolidated subsidiaries, which is referred to as Air Products, that converted to Versum RSU awards in connection with the spin-off of Versum from Air Products), each of which is referred to as a Versum RSU award, and Versum performance-based restricted stock unit award (including “market-based performance stock units,” each of which is referred to as a Versum MSU and “performance-based performance stock units,” each of which is referred to as a Versum PRSU) (collectively, Versum MSUs and Versum PRSUs are referred to as Versum PSU awards), relating to Versum common stock that is outstanding immediately prior to the effective time will be converted into a restricted stock unit award, each of which is referred to as a converted RSU award, relating to a number of shares of Entegris common stock equal to (i) the number of shares of Versum common stock subject to the Versum RSU award or Versum PSU award immediately prior to the effective time multiplied by (ii) the exchange ratio. For Versum PSU awards, the number of shares in clause (i) of the immediately preceding sentence will be determined based on actual performance through a shortened performance period ending immediately prior to the effective time. The converted RSU awards will vest and settle on terms (including acceleration events) at least as favorable as were applicable to the corresponding Versum RSU award or Versum PSU award immediately prior to the effective time, except that converted RSU awards relating to Versum PSU awards will vest solely based on continued service. Each converted RSU award will vest in full on the original vesting date or if the holder experiences a “covered termination” following the effective time and prior to the original vesting date, on the date of such “covered termination.” For employees of Versum and its subsidiaries, a “covered termination” means a termination by Versum or one of its subsidiaries or their successor in interest without “cause” or, to the extent good reason rights are provided for in the award agreement (or employment or similar agreement) applicable to the corresponding Versum RSU award or Versum PSU award, a resignation by the employee for “good reason” (as such terms are defined in the applicable Versum agreement), in either case, during the 24-month period following the effective time. For non-employee directors of Versum, a “covered termination” means the termination of their service as a director for any reason at or following the effective time.

At the effective time, each deferred stock unit award relating to shares of Versum common stock, each of which is referred to as a Versum DSU award, will vest and be converted into the right to receive a number of shares of Entegris common stock equal to (i) the number of shares of Versum common stock covered by the Versum DSU award immediately prior to the effective time by (ii) the exchange ratio, with such shares distributed to the holder within 30 business days following the effective time.

In considering the recommendations of the Entegris board of directors, Entegris stockholders should be aware that Entegris directors and executive officers have interests in the merger that are different from, or in addition to, their interests as Entegris stockholders. These interests may include, among others, the payment of severance benefits and acceleration of outstanding Entegris equity awards upon a qualifying termination

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of employment, and the payment of retention bonuses. For a more complete description of these interests, see the information provided in the section entitled “Interests of Entegris’ Directors and Executive Officers in the Merger” beginning on page 170.

In considering the recommendations of the Versum board of directors, Versum stockholders should be aware that Versum directors and executive officers have interests in the merger that are different from, or in addition to, their interests as Versum stockholders. These interests may include, among others, the payment of severance benefits and acceleration of outstanding Versum equity awards upon certain terminations of employment or service, the payment of retention bonuses and the combined company’s agreement to indemnify Versum directors and officers against certain claims and liabilities. For a more complete description of these interests, see the information provided in the section entitled “Interests of Versum’s Directors and Executive Officers in the Merger” beginning on page 176.

Entegris is commencing its solicitation of proxies on or about March 20, 2019, which is before the Entegris record date. Entegris will continue to solicit proxies until the date of the Entegris special meeting. Each holder of shares of Entegris common stock of record on April 2, 2019 who has not yet received this joint proxy statement/prospectus will receive this joint proxy statement/prospectus and have the opportunity to vote on the matters described in this joint proxy statement/prospectus. Proxies delivered prior to the Entegris record date will be valid and effective so long as the holder providing the proxy is a holder on the Entegris record date. If you are not a holder of record on the Entegris record date, any proxy you deliver will not be counted. If you deliver a proxy prior to the Entegris record date and remain a holder on the Entegris record date, you do not need to deliver another proxy after the Entegris record date. If you deliver a proxy prior to the Entegris record date and do not revoke that proxy, your proxy will be deemed to cover the number of shares of Entegris common stock you own on the Entegris record date even if that number is different from the number of shares of Entegris common stock you owned when you executed and delivered your WHITE proxy card.

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Versum is commencing its solicitation of proxies on or about March 20, 2019, which is before the Versum record date. Versum will continue to solicit proxies until the date of the Versum special meeting. Each holder of shares of Versum common stock of record on April 2, 2019 who has not yet received this joint proxy statement/prospectus will receive this joint proxy statement/prospectus and have the opportunity to vote on the matters described in this joint proxy statement/prospectus. Proxies delivered prior to the Versum record date will be valid and effective so long as the holder providing the proxy is a holder on the Versum record date. If you are not a holder of record on the Versum record date, any proxy you deliver will not be counted. If you deliver a proxy prior to the Versum record date and remain a holder on the Versum record date, you do not need to deliver another proxy after the Versum record date. If you deliver a proxy prior to the Versum record date and do not revoke that proxy, your proxy will be deemed to cover the number of shares of Versum common stock you own on the Versum record date even if that number is different from the number of shares of Versum common stock you owned when you executed and delivered your WHITE proxy card.

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A “broker non-vote” occurs on an item when (a) a bank, broker or other nominee has discretionary authority to vote on one or more proposals to be voted on at a meeting of stockholders, but is not permitted to vote on other proposals without instructions from the beneficial owner of the shares and (b) the beneficial owner fails to provide the bank, broker or other nominee with such instructions. Because none of the proposals currently scheduled to be voted on at either the Entegris special meeting or the Versum special meeting are routine matters for which brokers may have discretionary authority to vote, Entegris and Versum do not expect there to be any broker non-votes at the Entegris special meeting or the Versum special meeting.

Entegris Proposal 2: Entegris charter proposal. Assuming a quorum is present, the adoption of the amended and restated certificate of incorporation by Entegris stockholders requires the affirmative vote of a majority of the outstanding shares of Entegris common stock entitled to vote on such proposal. Accordingly, an Entegris stockholder’s abstention from voting, a broker non-vote or the failure of an Entegris stockholder to vote (including the failure of an Entegris stockholder who holds shares in “street name” through a bank, broker or other nominee to give voting instructions to that bank, broker or other nominee) will have the same effect as a vote “AGAINST” the Entegris charter proposal.

Entegris Proposal 3: Entegris compensation proposal. Assuming a quorum is present, approval of the Entegris compensation proposal requires the affirmative vote of a majority of votes properly cast on the

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proposal at the Entegris special meeting. Accordingly, an Entegris stockholder’s abstention from voting, a broker non-vote or an Entegris stockholder’s other failure to vote (including the failure of an Entegris stockholder who holds shares in “street name” through a bank, broker or other nominee to give voting instructions to that bank, broker or other nominee) will have no effect on the outcome of the Entegris compensation proposal.

Entegris Proposal 4: Entegris adjournment proposal. The Entegris special meeting may be adjourned to solicit additional proxies if there are not sufficient votes at the time of the Entegris special meeting to approve the Entegris merger agreement proposal and the Entegris charter proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to Entegris stockholders. Whether or not a quorum is present, the approval of the holders of a majority of the voting shares of Entegris common stock present or represented at the Entegris special meeting is required to adjourn the Entegris special meeting. An Entegris stockholder’s abstention from voting will have the same effect as a vote “AGAINST” the Entegris adjournment proposal, while a broker non-vote or the failure of an Entegris stockholder to vote (including the failure of an Entegris stockholder who holds shares in “street name” through a bank, broker or other nominee to give voting instructions to that bank, broker or other nominee) will have no effect on the Entegris adjournment proposal.

Versum Proposal 2: Versum compensation proposal. Assuming a quorum is present, approval of the Versum compensation proposal requires the affirmative vote of a majority of the votes cast at the Versum special meeting on this proposal. Accordingly, a Versum stockholder’s abstention from voting, a broker non-vote or the failure of a Versum stockholder to vote (including the failure of a Versum stockholder who holds shares in “street name” through a bank, broker or other nominee to give voting instructions to that bank, broker or other nominee) will have no effect on the Versum compensation proposal.

Versum Proposal 3: Versum adjournment proposal. The Versum special meeting may be adjourned to solicit additional proxies if there are not sufficient votes at the time of the Versum special meeting to approve the Versum merger agreement proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to Versum stockholders. Whether or not a quorum is present, the affirmative vote of a majority of the votes entitled to be cast who are present in person or represented by proxy at the Versum special meeting is required to adjourn the Versum special meeting. Accordingly, a Versum stockholder’s abstention from voting will have the same effect as a vote “AGAINST” the Versum adjournment proposal, while a broker non-vote or the failure of a Versum stockholder to vote (including the failure of a Versum stockholder who holds shares in “street name” through a bank, broker or other nominee to give voting instructions to that bank, broker or other nominee) will have no effect on the Versum adjournment proposal. The chairman of the Versum special meeting may also adjourn the Versum special meeting, whether or not there is a quorum.

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Shares in “street name.” Shares held in “street name” may be voted in person by you only if you obtain a signed legal proxy from your bank, broker or other nominee giving you the right to vote the shares. If you choose to vote your shares in person at the Entegris special meeting or Versum special meeting, as applicable, please bring required documentation in accordance with the section entitled “The Entegris Special Meeting—Attending the Entegris Special Meeting” beginning on page 63, with respect to the Entegris special meeting, and the section entitled “The Versum Special Meeting—Attending the Versum Special Meeting” beginning on page 74, with respect to the Versum special meeting.

Even if you plan to attend the Entegris special meeting or the Versum special meeting, as applicable, Entegris and Versum recommend that you vote your shares in advance as described below so that your vote will be counted if you later decide not to or become unable to attend the respective special meeting. The use of video, still photography or audio recording is not permitted at either special meeting. For the safety of attendees, all bags, packages and briefcases are subject to inspection. Your compliance is appreciated.

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Additional information on attending the special meetings can be found under the section entitled “The Entegris Special Meeting” on page 59 and under the section entitled “The Versum Special Meeting” on page 70.

Additional information on voting procedures can be found under the section entitled “The Entegris Special Meeting” on page 59 and under the section entitled “The Versum Special Meeting” on page 70.

If your shares of common stock in Entegris or Versum are held by a bank, broker or other nominee, you are considered the beneficial owner of shares held in “street name,” and your bank, broker or other nominee is considered the stockholder of record with respect to those shares. Your bank, broker or other nominee will send you, as the beneficial owner, a package describing the procedure for voting your shares. You should follow the instructions provided by them to vote your shares. You are invited to attend the Entegris special meeting or the Versum special meeting, as applicable, however, you may not vote these shares in person at the respective special meeting unless you obtain a signed legal proxy, executed in your favor, from your bank, broker or other nominee that holds your shares, giving you the right to vote the shares at the applicable special meeting.

For Entegris stockholders, the effect of not instructing your bank, broker or other nominee how you wish to vote your shares will be the same as a vote “AGAINST” the Entegris merger agreement proposal and the

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Entegris charter proposal, but will not be counted as “FOR” or “AGAINST” and, assuming a quorum is present at the Entegris special meeting, will have no effect on, the Entegris compensation proposal or the Entegris adjournment proposal.

For Versum stockholders, the effect of not instructing your bank, broker or other nominee how you wish to vote your shares will be the same as a vote “AGAINST” the Versum merger agreement proposal, but will not be counted as “FOR” or “AGAINST” and, assuming a quorum is present at the Versum special meeting, will have no effect on, the Versum compensation proposal or the Versum adjournment proposal.

Record Holders. For shares held directly, please complete, sign, date and return each WHITE proxy card (or cast your vote by telephone or Internet as provided on each WHITE proxy card) or otherwise follow the voting instructions provided in this joint proxy statement/prospectus in order to ensure that all of your shares of Entegris common stock or Versum common stock are voted.

Shares in “street name.” For shares held in “street name” through a bank, broker or other nominee, you should follow the procedures provided by your bank, broker or other nominee to vote your shares.

We urge you to discard any green proxy cards and disregard any related solicitation materials, which may have been sent to you by Merck KGaA, Darmstadt, Germany, which is referred to as Merck, who is soliciting proxies from Versum stockholders in opposition to the merger. If you previously submitted a green proxy card, we urge you to cast your vote as instructed on your WHITE proxy card, which will revoke any earlier dated proxy card that you may have submitted, including any green proxy card. Only the latest dated proxy you submit will be counted.

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Execution or revocation of a proxy will not in any way affect your right to attend the applicable special meeting and vote in person. Written notices of revocation and other communications with respect to the revocation of proxies should be addressed:

For more information, see the section entitled “The Entegris Special Meeting—Revocability of Proxies” beginning on page 63 and the section entitled “The Versum Special Meeting—Revocability of Proxies” beginning on page 73, as applicable.

If your shares are held in the name of a bank, broker or other nominee and you previously provided voting instructions to your bank, broker or other nominee, you should follow the instructions provided by your bank, broker or other nominee to revoke or change your voting instructions.

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For further information, see the section entitled “Material U.S. Federal Income Tax Consequences” beginning on page 183.

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Entegris and Versum have agreed to coordinate with each other on the declaration, setting of record dates and payment dates of dividends with the intention that holders of shares of Versum common stock do not receive dividends on both shares of Versum common stock and Entegris common stock received in the merger in respect of any calendar quarter or fail to receive a dividend on either shares of Versum common stock or Entegris common stock received in the merger in respect of any calendar quarter. The declaration and payment of any dividends on shares of Entegris common stock or Versum common stock remain subject to applicable law and the approval of the Versum board of directors or the Entegris board of directors, as applicable.

For more information regarding the payment of dividends, see the section entitled “The Merger Agreement—Conduct of Business Prior to the Effective Time” beginning on page 136.

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RISK FACTORS

In deciding whether to vote for the adoption of the merger agreement and the adoption of the amended and restated certificate of incorporation of Entegris, in the case of Entegris stockholders, or the adoption of the merger agreement, in the case of Versum stockholders, you are urged to carefully consider all of the information included or incorporated by reference in this joint proxy statement/prospectus, which is listed in the section entitled “Where You Can Find More Information” beginning on page 213. You should also read and consider the risks associated with each of the businesses of Entegris and Versum because these risks will also affect the combined company. The risks associated with the business of Entegris can be found in Entegris’ Annual Report on Form 10-K for the fiscal year ended December 31, 2018 and the risks associated with the business of Versum can be found in Versum’s Annual Report on Form 10-K for the year ended September 30, 2018, as such risks may be updated or supplemented in each company’s subsequently filed Quarterly Reports on Form 10-Q or Current Reports on Form 8-K (excluding any information and exhibits furnished under Item 2.02 or 7.01 thereof), each of which are incorporated by reference into this joint proxy statement/prospectus. In addition, you are urged to carefully consider the following material risks relating to the merger, the business of Entegris, the business of Versum and the business of the combined company.

Risks Relating to the Merger

Because the exchange ratio is fixed and will not be adjusted in the event of any change in either Entegris’ or Versum’s stock price, the value of the shares of the combined company is uncertain.

Upon completion of the merger, each share of Versum common stock outstanding immediately prior to the merger, other than Versum excluded shares, will be converted into and become exchangeable for 1.120 shares of Entegris common stock. This exchange ratio is fixed in the merger agreement and will not be adjusted for changes in the market price of either Entegris common stock or Versum common stock. The market prices of Entegris common stock and Versum common stock have fluctuated prior to and after the date of the announcement of the merger agreement and will continue to fluctuate from the date of this joint proxy statement/prospectus to the date of the Entegris special meeting and the Versum special meeting, respectively, and the date the merger is consummated, and the market price of the common stock of the combined company will continue to fluctuate thereafter.

Because the value of the merger consideration will depend on the market price of Entegris common stock at the time the merger is completed, Versum stockholders will not know or be able to determine at the time of the Versum special meeting the market value of the merger consideration they would receive upon completion of the merger. Similarly, Entegris stockholders will not know or be able to determine at the time of the Entegris special meeting the market value of the shares of Entegris common stock to be issued pursuant to the merger agreement compared to the market value of the shares of Versum common stock that are being exchanged in the merger.

Stock price changes may result from a variety of factors, including, among others, general market and economic conditions, changes in Entegris’ or Versum’s respective businesses, operations and prospects, reductions or changes in U.S. government spending or budgetary policies, market assessments of the likelihood that the merger will be completed, interest rates, general market, industry and economic conditions and other factors generally affecting the respective prices of Entegris’ or Versum’s common stock, federal, state and local legislation, governmental regulation and legal developments in the industry segments in which Versum or Entegris operate, and the timing of the merger and receipt of required regulatory approvals.

Many of these factors are beyond Entegris’ and Versum’s control, and neither Entegris nor Versum are permitted to terminate the merger agreement solely due to a decline in the market price of the common stock of the other party. You are urged to obtain current market quotations for Entegris common stock and Versum common stock in determining whether to vote for the adoption of the merger agreement and the adoption of the amended and restated certificate of incorporation of Entegris in the case of Entegris stockholders or for the

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adoption of the merger agreement in the case of Versum stockholders. In addition, see the section entitled “Comparison of Entegris and Versum Market Prices and Implied Value of Merger Consideration” beginning on page 40.

The merger may not be completed and the merger agreement may be terminated in accordance with its terms.

The merger is subject to a number of conditions that must be satisfied, including the approval by Entegris stockholders of the Entegris merger agreement proposal and approval by Versum stockholders of the Versum merger agreement proposal, or waived (to the extent permissible), in each case prior to the completion of the merger. These conditions are described in the section entitled “The Merger Agreement—Conditions to the Completion of the Merger” beginning on page 152. These conditions to the completion of the merger, some of which are beyond the control of Entegris and Versum, may not be satisfied or waived in a timely manner or at all, and, accordingly, the merger may be delayed or not completed.

Additionally, either Entegris or Versum may terminate the merger agreement under certain circumstances, including, among other reasons, if the merger is not completed by January 28, 2020 (which date may be extended to April 28, 2020 under certain circumstances if certain regulatory approvals are not obtained by January 28, 2020). In addition, if the merger agreement is terminated under certain circumstances specified in the merger agreement, Versum may be required to pay Entegris a termination fee of $140 million, including certain circumstances in which the Versum board of directors effects a change of recommendation (as defined in the section entitled “The Merger Agreement—No Change of Recommendation” beginning on page 142) or Versum enters into an agreement with respect to a superior proposal following the termination of the merger agreement. In addition, if the merger agreement is terminated under certain circumstances specified in the merger agreement, Entegris may be required to pay Versum a termination fee of $155 million, including certain circumstances in which the Entegris board of directors effects a change of recommendation or Entegris enters into an agreement with respect to a superior proposal following the termination of the merger agreement. See the section entitled “The Merger Agreement—Termination of the Merger Agreement” beginning on page 153 and the section entitled “The Merger Agreement—Termination Fees” beginning on page 155 for a more complete discussion of the circumstances under which the merger agreement could be terminated and when a termination fee may be payable by Entegris or Versum.

The termination of the merger agreement could negatively impact Entegris or Versum.

If the merger is not completed for any reason, including as a result of a failure to obtain the required Versum vote or the required Entegris vote, the ongoing businesses of Entegris and Versum may be adversely affected and, without realizing any of the benefits of having completed the merger, Entegris and Versum would be subject to a number of risks, including the following:

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The market price for shares of common stock of the combined company following the completion of the merger may be affected by factors different from, or in addition to, those that historically have affected or currently affect the market prices of shares of Entegris common stock and Versum common stock.

Upon consummation of the merger, Entegris stockholders and Versum stockholders will both hold shares of common stock in the combined company. Entegris’ businesses differ from those of Versum, and Versum’s businesses differ from those of Entegris, and, accordingly, the results of operations of the combined company will be affected by some factors that are different from those currently or historically affecting the results of operations of Entegris and those currently or historically affecting the results of operations of Versum. The results of operations of the combined company may also be affected by factors different from those that currently affect or have historically affected either Entegris or Versum. For a discussion of the businesses of each of Entegris and Versum and some important factors to consider in connection with those businesses, please see the section entitled “The Parties to the Merger” beginning on page 58 and the documents and information included elsewhere in this joint proxy statement/prospectus or incorporated by reference into this joint proxy statement/prospectus and listed under the section entitled “Where You Can Find More Information” beginning on page 213.

The shares of common stock of the combined company to be received by Versum stockholders as a result of the merger will have rights different from the shares of Versum common stock.

Upon consummation of the merger, the rights of Versum stockholders, who will become stockholders of the combined company, will be governed by the certificate of incorporation and bylaws of the combined company. The rights associated with Versum common stock are different from the rights which will be associated with the common stock of the combined company. See the section entitled “Comparison of Stockholders’ Rights” beginning on page 188 for a discussion of these rights.

Entegris stockholders and Versum stockholders will each have reduced ownership and voting interest in and will exercise less influence over management of the combined company.

Entegris stockholders currently have the right to vote in the election of the Entegris board of directors and on other matters affecting Entegris, and Versum stockholders currently have the right to vote in the election of the Versum board of directors and on other matters affecting Versum. Upon consummation of the merger, each Entegris stockholder and each Versum stockholder will become a stockholder of the combined company with a percentage ownership of the combined company that is smaller than such stockholder’s percentage ownership of Entegris or Versum, as applicable, immediately prior to the merger. As of the date of this joint proxy statement/prospectus, based on the exchange ratio of 1.120 and the estimated number of shares of common stock of Entegris and Versum that will be outstanding immediately prior to the completion of the merger, including exercisable options, Entegris and Versum estimate that holders of shares of Entegris common stock as of immediately prior to the completion of the merger will hold, in the aggregate, approximately 52.5% of the issued and outstanding shares of common stock of the combined company (based on fully diluted shares outstanding of the combined company including exercisable options only) immediately following the completion of the merger, and holders of shares of Versum common stock as of immediately prior to the completion of the merger will hold, in the aggregate, approximately 47.5% of the issued and outstanding shares of common stock of the combined company (based on fully diluted shares outstanding of the combined company including exercisable options only) immediately following the completion of the merger. Because of this, each share of Entegris common stock and each share of Versum common stock will represent a smaller percentage ownership of the combined company than it represented in Entegris or Versum, respectively. In addition, directors of Entegris and

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directors of Versum, in each case, as of immediately prior to the effective time, who will be determined prior to closing by mutual agreement of the parties, will respectively represent five and four of the nine members of the combined company’s board of directors as of the effective time. Accordingly, each Entegris stockholder and each Versum stockholder will have less influence on the management and policies of the combined company than such stockholder now has on the management and policies of Entegris or Versum, as applicable.

Until the completion of the merger or the termination of the merger agreement in accordance with its terms, Entegris and Versum are each prohibited from entering into certain transactions and taking certain actions that might otherwise be beneficial to Entegris or Versum and their respective stockholders.

From and after the date of the merger agreement and prior to completion of the merger, the merger agreement restricts Entegris and Versum from taking specified actions without the consent of the other party and requires that the business of each company and its respective subsidiaries be conducted in all material respects in the ordinary course of business consistent with past practice. These restrictions may prevent Entegris or Versum from making appropriate changes to their respective businesses or organizational structures or from pursuing attractive business opportunities that may arise prior to the completion of the merger, and could have the effect of delaying or preventing other strategic transactions. Adverse effects arising from the pendency of the merger could be exacerbated by any delays in consummation of the merger or termination of the merger agreement. See the section entitled “The Merger Agreement—Conduct of Business Prior to the Effective Time” beginning on page 136.

Obtaining required approvals and satisfying closing conditions may prevent or delay completion of the merger.

The merger is subject to a number of conditions to closing as specified in the merger agreement. These closing conditions include, among others, approval for listing on NASDAQ or the NYSE of the shares of Entegris common stock to be issued pursuant to the merger agreement, the expiration or earlier termination of any applicable waiting period, and the receipt of approvals under, U.S. and foreign antitrust and competition laws in China, Germany, Japan, South Korea, and Taiwan, the absence of governmental restraints or prohibitions preventing the consummation of the merger, the effectiveness of the registration statement on Form S-4 registering the Entegris common stock issuable pursuant to the merger agreement and the absence of any stop order or proceedings by the SEC with respect thereto. The obligation of each of Versum and Entegris to consummate the merger is also conditioned on, among other things, the receipt by such party of a written opinion from such party’s counsel, to the effect that for U.S. federal income tax purposes the merger will qualify as a “reorganization” within the meaning of Section 368(a) of the Code, the absence of a material adverse effect on the other party, the truth and correctness of the representations and warranties made by the other party on the date of the merger agreement and on the closing date (subject to certain materiality qualifiers), and the performance by the other party in all material respects of its obligations under the merger agreement. No assurance can be given that the required stockholder, governmental and regulatory consents and approvals will be obtained or that the required conditions to closing will be satisfied, and, if all required consents and approvals are obtained and the conditions are satisfied, no assurance can be given as to the terms, conditions and timing of such consents and approvals. Any delay in completing the merger could cause the combined company not to realize, or to be delayed in realizing, some or all of the benefits that Entegris and Versum expect to achieve if the merger is successfully completed within its expected time frame. For a more complete summary of the conditions that must be satisfied or waived prior to completion of the merger, see the section entitled “The Merger Agreement—Conditions to the Completion of the Merger” beginning on page 152.

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Entegris and Versum must obtain certain regulatory approvals and clearances to consummate the merger, which, if delayed, not granted or granted with unacceptable conditions, could prevent, substantially delay or impair consummation of the merger, result in additional expenditures of money and resources or reduce the anticipated benefits of the merger.

The completion of the merger is subject to the receipt of antitrust clearance in the United States and in China, Germany, Japan, South Korea and Taiwan.

With respect to the United States, under the HSR Act, the merger may not be completed until Notification and Report Forms have been filed with the FTC and the DOJ and the applicable waiting period has expired or been terminated. A transaction requiring notification under the HSR Act may not be completed until the expiration of a 30-calendar-day waiting period following the parties’ filing of their respective HSR notifications or the early termination of that waiting period. Entegris and Versum each filed an HSR notification with the FTC and the DOJ on February 6, 2019 and the waiting period expired at 11:59 p.m. Eastern Time on March 8, 2019. With respect to Germany, clearance was received on March 19, 2019.

At any time before or after consummation of the merger, notwithstanding the expiration or termination of the applicable waiting period under the HSR Act, the DOJ or the FTC, or any state, could take such action under the antitrust laws as it deems necessary or desirable in the public interest, including seeking to enjoin the completion of the merger, seeking divestiture of substantial assets of the parties or requiring the parties to license, or hold separate, assets or terminate existing relationships and contractual rights. At any time before or after the completion of the merger, and notwithstanding the expiration or termination of the applicable waiting period under the HSR Act, any state could take such action under the antitrust laws as it deems necessary or desirable in the public interest. Such action could include seeking to enjoin the completion of the merger or seeking divestiture of substantial assets of the parties. Private parties may also seek to take legal action under the antitrust laws under certain circumstances.

The merger is also subject to clearance or approval by antitrust authorities in certain other jurisdictions. The merger cannot be completed until Entegris and Versum obtain clearance to consummate the merger or applicable waiting periods have expired or been terminated in each applicable jurisdiction. Entegris and Versum, in consultation and cooperation with each other, will file notifications, as required with antitrust authorities in certain other jurisdictions, as promptly as practicable after the date of the merger agreement. The relevant antitrust authorities could take such actions under the applicable antitrust laws as they deem necessary or desirable, including seeking divestiture of substantial assets of the parties or requiring the parties to license, or hold separate, assets or terminate existing relationships and contractual rights. Any one of these requirements, limitations, costs, divestitures or restrictions could jeopardize or delay the completion of or reduce the anticipated benefits of the merger. There is no assurance that Entegris and Versum will obtain all required antitrust clearances or approvals on a timely basis, or at all. Failure to obtain the necessary clearance in any of these jurisdictions could substantially delay or prevent the consummation of the merger, which could negatively impact both Entegris and Versum.

Failure to attract, motivate and retain executives and other key employees could diminish the anticipated benefits of the merger.

The success of the merger will depend in part on the retention of personnel critical to the business and operations of the combined company due to, for example, their technical skills or management expertise. Competition for qualified personnel can be intense.

Current and prospective employees of Entegris and Versum may experience uncertainty about their future role with Entegris and Versum until strategies with regard to these employees are announced or executed, which may impair Entegris’ and Versum’s ability to attract, retain and motivate key management, sales, marketing, technical and other personnel prior to and following the merger. Employee retention may be particularly challenging during the pendency of the merger, as employees of Entegris and Versum may experience

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uncertainty about their future roles with the combined company. If Entegris and Versum are unable to retain personnel, including Entegris’ and Versum’s key management, who are critical to the successful integration and future operations of the companies, Entegris and Versum could face disruptions in their operations, loss of existing customers or loss of sales to existing customers, loss of key information, expertise or know-how, and unanticipated additional recruitment and training costs. In addition, the loss of key personnel could diminish the anticipated benefits of the merger.

If key employees of Entegris or Versum depart, the integration of the companies may be more difficult and the combined company’s business following the merger may be harmed. Furthermore, the combined company may have to incur significant costs in identifying, hiring and retaining replacements for departing employees and may lose significant expertise and talent relating to the business of each of Entegris or Versum, and the combined company’s ability to realize the anticipated benefits of the merger may be adversely affected. In addition, there could be disruptions to or distractions for the workforce and management associated with activities of labor unions or integrating employees into the combined company. No assurance can be given that the combined company will be able to attract or retain key employees of Entegris and Versum to the same extent that those companies have been able to attract or retain their own employees in the past.

The merger, and uncertainty regarding the merger, may cause customers, suppliers or strategic partners to delay or defer decisions concerning Entegris and Versum and adversely affect each company’s ability to effectively manage their respective businesses.

The merger will happen only if the stated conditions are met, including the adoption of the merger agreement by Entegris stockholders and Versum stockholders and the receipt of regulatory approvals, among other conditions. Many of the conditions are outside the control of Entegris and Versum, and both parties also have certain rights to terminate the merger agreement. Accordingly, there may be uncertainty regarding the completion of the merger. This uncertainty may cause customers, suppliers, vendors, strategic partners or others that deal with Entegris or Versum to delay or defer entering into contracts with Entegris or Versum or making other decisions concerning Entegris or Versum or seek to change or cancel existing business relationships with Entegris or Versum, which could negatively affect their respective businesses. Any delay or deferral of those decisions or changes in existing agreements could have an adverse impact on the respective businesses of Entegris and Versum, regardless of whether the merger is ultimately completed.

In addition, the merger agreement restricts Entegris, Versum and their respective subsidiaries from making certain acquisitions and taking other specified actions until the merger occurs without the consent of the other parties. These restrictions may prevent Entegris and Versum from pursuing attractive business opportunities or strategic transactions that may arise prior to the completion of the merger. See the section entitled “The Merger Agreement—Conduct of Business Prior to the Effective Time” beginning on page 136 for a description of the restrictive covenants to which each of Entegris and Versum is subject.

The opinions rendered to Entegris and Versum from their respective financial advisors will not reflect changes in circumstances between the dates of such opinions and the completion of the merger.

Morgan Stanley delivered its oral opinion to the Entegris board of directors on January 27, 2019, which opinion was subsequently confirmed in a written opinion dated January 27, 2019, that as of such date and based upon and subject to the various assumptions made, procedures followed, matters considered, and qualifications and limitations on the scope of the review undertaken by Morgan Stanley as set forth in the written opinion, the exchange ratio pursuant to the merger agreement was fair from a financial point of view to Entegris. Lazard rendered an oral opinion to the Versum board of directors on January 27, 2019, subsequently confirmed in writing by delivery of Lazard’s opinion dated as of the same date, to the effect that, as of such date, and based upon and subject to the assumptions, procedures, factors, qualifications and limitations set forth in its written opinion, the exchange ratio in the merger was fair, from a financial point of view, to Versum stockholders (other than the holders of Versum excluded shares).

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Neither Entegris nor Versum has obtained, nor will obtain, an updated opinion regarding the fairness, from a financial point of view, of the exchange ratio as of the date of this joint proxy statement/prospectus or prior to the completion of the merger from Morgan Stanley or from Lazard. Each of Morgan Stanley’s opinion and Lazard’s opinion was necessarily based on economic, monetary, market and other conditions as in effect on, and the information made available to Morgan Stanley and Lazard, as applicable, only as of the dates of the respective opinions of Morgan Stanley and Lazard and does not address the fairness of the exchange ratio, from a financial point of view, at the time the merger is completed. Changes in the operations and prospects of Entegris or Versum, general economic, monetary, market and other conditions and other factors that may be beyond the control of Entegris and Versum, and on which the opinion of Morgan Stanley and the opinion of Lazard was based, may alter the value of Entegris or Versum or the prices of shares of Entegris common stock or Versum common stock by the time the merger is completed. The opinions of Morgan Stanley and Lazard do not speak as of any date other than the respective dates of such opinions. The recommendation of the Entegris board of directors that Entegris stockholders vote “FOR” the Entegris merger agreement proposal, “FOR” the Entegris charter proposal, “FOR” the Entegris compensation proposal and “FOR” the Entegris adjournment proposal and the recommendation of the Versum board of directors that Versum stockholders vote “FOR” the Versum merger agreement proposal, “FOR” the Versum compensation proposal and “FOR” the Versum adjournment proposal are each made as of the date of this joint proxy statement/prospectus. For a description of the opinion that Versum and Entegris received from their respective financial advisors, please see the sections entitled “The Merger—Opinion of Entegris’ Financial Advisor” beginning on page 96 and “The Merger—Opinion of Versum’s Financial Advisor” beginning on page 107.

Whether or not the merger is completed, the announcement and pendency of the merger could cause disruptions in the businesses of Entegris and Versum, which could have an adverse effect on their respective businesses and financial results.

Whether or not the merger is completed, the announcement and pendency of the merger could cause disruptions in the businesses of Entegris and Versum. Specifically:

In addition, Entegris and Versum have each diverted significant management resources in an effort to complete the merger and are each subject to restrictions contained in the merger agreement on the conduct of their respective businesses. If the merger is not completed, Entegris and Versum will have incurred significant costs, including the diversion of management resources, for which they will have received little or no benefit.

The directors and executive officers of Entegris and Versum have interests and arrangements that may be different from, or in addition to, those of Entegris and Versum stockholders generally.

When considering the recommendations of the boards of directors of Entegris or Versum, as applicable, with respect to the proposals described in this joint proxy statement/prospectus, stockholders should be aware that the directors and executive officers of each of Entegris and Versum have interests in the merger that are different from, or in addition to, those of Entegris stockholders and Versum stockholders generally. These interests include the continued employment of certain executive officers of Entegris and Versum by the combined company, the continued service of certain directors of Entegris and Versum as directors of the combined company, the treatment in the merger of outstanding equity, equity-based and incentive awards, severance arrangements, other compensation and benefit arrangements, and the right to continued indemnification of former Entegris and Versum directors and officers by the combined company.

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Entegris stockholders and Versum stockholders should be aware of these interests when they consider the recommendations of the respective Entegris and Versum boards of directors that they vote to adopt the merger agreement and to adopt the amended and restated certificate of incorporation of Entegris, in the case of Entegris, or that they adopt the merger agreement, in the case of Versum. The Entegris board of directors was aware of these interests when it approved and declared advisable the merger agreement and the transactions contemplated thereby on the terms and subject to the conditions set forth in the merger agreement and recommended that Entegris stockholders adopt the merger agreement and adopt the amended and restated certificate of incorporation of Entegris. The interests of Entegris directors and executive officers are described in more detail in the section entitled “Interests of Entegris’ Directors and Executive Officers in the Merger” beginning on page 170. Likewise, the Versum board of directors was aware of these interests when it approved and declared advisable the merger agreement, the merger and the transactions contemplated thereby on the terms and subject to the conditions set forth in the merger agreement, determined that the merger agreement, the merger and the transactions contemplated by the merger agreement were fair to, and in the best interests of, Versum and Versum stockholders and recommended that Versum stockholders adopt the merger agreement. The interests of Versum directors and executive officers are described in more detail in the section entitled “Interests of Versum’s Directors and Executive Officers in the Merger” beginning on page 176.

Entegris or Versum may waive one or more of the closing conditions without re-soliciting stockholder approval.

Entegris or Versum may determine to waive, in whole or part, one or more of the conditions of its obligations to consummate the merger. Entegris and Versum currently expect to evaluate the materiality of any waiver and its effect on Entegris or Versum stockholders, as applicable, in light of the facts and circumstances at the time to determine whether any amendment of this joint proxy statement/prospectus or any re-solicitation of proxies or voting cards is required in light of such waiver. Any determination whether to waive any condition to the merger or as to re-soliciting stockholder approval or amending this joint proxy statement/prospectus as a result of a waiver will be made by Entegris or Versum, as applicable, at the time of such waiver based on the facts and circumstances as they exist at that time.

The merger agreement contains provisions that could discourage a potential competing acquirer that might be willing to pay more to acquire or merge with either Entegris or Versum.

The merger agreement contains “no shop” provisions that restrict each of Entegris’ and Versum’s ability to, among other things (each as described under the section entitled “The Merger Agreement—No Solicitation of Acquisition Proposals” beginning on page 140):

Furthermore, there are only limited exceptions to the requirement under the merger agreement that neither the Entegris board of directors nor the Versum board of directors adversely withhold, withdraw, qualify or

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modify the Entegris recommendation or the Versum recommendation, as applicable (each as defined in the section entitled “The Merger Agreement—Representations and Warranties” beginning on page 133). Although the Entegris board of directors is permitted to effect a change of recommendation, after complying with certain procedures set forth in the merger agreement, in response to a superior proposal or an intervening event, if it determines in good faith that a failure to do so would be inconsistent with its fiduciary duties, such change of recommendation would entitle Versum to terminate the merger agreement and collect a termination fee from Entegris in the amount of $155 million. Although the Versum board of directors is permitted to effect a change of recommendation, after complying with certain procedures set forth in the merger agreement, in response to a superior proposal or an intervening event, if it determines in good faith that a failure to do so would be inconsistent with its fiduciary duties, such change of recommendation would entitle Entegris to terminate the merger agreement and collect a termination fee from Versum in the amount of $140 million. For more information, see the sections titled “The Merger Agreement—Termination of the Merger Agreement” beginning on page 153 and “The Merger Agreement—Termination Fees” beginning on page 155.

These provisions could discourage a potential competing acquirer from considering or proposing an acquisition or merger, even if it were prepared to pay consideration with a higher value than that implied by the exchange ratio in the merger, or might result in a potential competing acquirer proposing to pay a lower per share price than it might otherwise have proposed to pay because of the added expense of the termination fee.

Each of Entegris and Versum will incur significant transaction, merger-related and restructuring costs in connection with the merger.

Entegris and Versum have incurred and expect to incur a number of non-recurring costs associated with combining the operations of the two companies, as well as transaction fees and other costs related to the merger. These costs and expenses include fees paid to financial, legal and accounting advisors, facilities and systems consolidation costs, severance and other potential employment-related costs, including retention and severance payments that may be made to certain Entegris employees and Versum employees, filing fees, printing expenses and other related charges. Some of these costs are payable by Entegris or Versum regardless of whether the merger is completed.

The combined company will also incur restructuring and integration costs in connection with the merger. The costs related to restructuring will be expensed as a cost of the ongoing results of operations of either Entegris or Versum or the combined company. There are a large number of processes, policies, procedures, operations, technologies and systems that must be integrated in connection with the merger and the integration of the two companies’ businesses. Although Entegris and Versum expect that the elimination of duplicative costs, strategic benefits, additional income as well as the realization of other efficiencies related to the integration of the businesses, may offset incremental transaction, merger-related and restructuring costs over time, any net benefit may not be achieved in the near term or at all. Many of these costs will be borne by Entegris or Versum even if the merger is not completed. While both Entegris and Versum have assumed that certain expenses would be incurred in connection with the merger and the other transactions contemplated by the merger agreement, there are many factors beyond their control that could affect the total amount or the timing of the integration and implementation expenses.

Entegris stockholders and Versum stockholders will not be entitled to appraisal rights in the merger.

Appraisal rights are statutory rights that, if applicable under law, enable stockholders of a corporation to dissent from an extraordinary transaction, such as a merger, and to demand that such corporation pay the fair value for their shares as determined by a court in a judicial proceeding instead of receiving the consideration offered to such stockholders in connection with the transaction. Under the DGCL, stockholders do not have appraisal rights if the shares of stock they hold are either listed on a national securities exchange or held of record by more than 2,000 holders. Notwithstanding the foregoing, appraisal rights are available if stockholders are required by the terms of the merger agreement to accept for their shares anything other than (a) shares of stock of

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the surviving corporation, (b) shares of stock of another corporation that will either be listed on a national securities exchange or held of record by more than 2,000 holders, (c) cash in lieu of fractional shares or (d) any combination of the foregoing.

Because the merger is of Versum with and into Entegris and holders of Entegris common stock will continue to hold their shares following completion of the merger, holders of Entegris common stock are not entitled to appraisal rights in the merger.

Because shares of Entegris common stock are listed on NASDAQ, a national securities exchange, and are expected to continue to be so listed or listed on the NYSE, a national securities exchange, and because Versum stockholders are not required by the terms of the merger agreement to accept for their shares anything other than shares of Entegris common stock and cash in lieu of fractional shares, holders of Versum common stock will not be entitled to appraisal rights in the merger. See the section entitled “No Appraisal Rights” beginning on page 204.

Litigation filed against Versum, the Versum board of directors and Entegris could prevent or delay the consummation of the merger or result in the payment of damages following completion of the merger.

In connection with the merger, purported stockholders of Versum have filed five putative class action lawsuits and one individual lawsuit against one or more of Versum, the members of the Versum board of directors, Entegris and Broadridge Corporate Issuer Solutions, Inc. Among other remedies, the plaintiffs in these lawsuits seek to enjoin the merger and any vote on the merger. The outcome of any such litigation is uncertain. If a dismissal is not granted or a settlement is not reached, these lawsuits could prevent or delay completion of the merger and result in substantial costs to Versum and Entegris, including any costs associated with indemnification. Additional lawsuits in connection with the merger may be filed against Versum, Entegris and/or their respective directors and officers, which additional lawsuits could also prevent or delay the consummation of the merger and result in additional costs to Versum and Entegris. The ultimate resolution of these lawsuits cannot be predicted with certainty, and an adverse ruling in any such lawsuit may cause the merger to be delayed or not to be completed, which could cause Versum and Entegris not to realize some or all of the anticipated benefits of the merger. The defense or settlement of any lawsuit or claim that remains unresolved at the time the merger is consummated may adversely affect the combined company’s business, financial condition, results of operations and cash flows. Versum and Entegris cannot currently predict the outcome of or reasonably estimate the possible loss or range of loss from any these lawsuits or claims. See “The Merger—Litigation Related to the Merger” beginning on page 126 for more information about the lawsuits that have been filed related to the merger.

Risks Relating to the Combined Company

The failure to successfully combine the businesses of Entegris and Versum may adversely affect the combined company’s future results.

The success of the merger will depend, in part, on the ability of the combined company to realize anticipated benefits from combining the businesses of Entegris and Versum. To realize these anticipated benefits, the businesses of Entegris and Versum must be successfully combined. If the combined company is not able to achieve these objectives, the anticipated benefits of the merger may not be realized fully or at all, or may take longer to realize than expected.

The combined company may not be able to retain customers or suppliers or customers or suppliers may seek to modify contractual obligations with the combined company, which could have an adverse effect on the combined company’s business and operations. Third parties may terminate or alter existing contracts or relationships with Entegris or Versum.

As a result of the merger, the combined company may experience impacts on relationships with customers and suppliers that may harm the combined company’s business and results of operations. Certain customers or suppliers may seek to terminate or modify contractual obligations following the merger whether or not

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contractual rights are triggered as a result of the merger. There can be no guarantee that customers and suppliers will remain with or continue to have a relationship with the combined company or do so on the same or similar contractual terms following the merger. If any customers or suppliers seek to terminate or modify contractual obligations or discontinue the relationship with the combined company, then the combined company’s business and results of operations may be harmed. Furthermore, the combined company will not have long-term arrangements with many of its significant suppliers. If the combined company’s suppliers were to seek to terminate or modify an arrangement with the combined company, then the combined company may be unable to procure necessary supplies from other suppliers in a timely and efficient manner and on acceptable terms, or at all.

Entegris and Versum also have contracts with vendors, landlords, licensors and other business partners which may require Entegris or Versum, as applicable, to obtain consent from these other parties in connection with the merger. If these consents cannot be obtained, the combined company may suffer a loss of potential future revenue, incur costs, and lose rights that may be material to the business of the combined company. In addition, third parties with whom Entegris or Versum currently have relationships may terminate or otherwise reduce the scope of their relationship with either party in anticipation of the merger. Any such disruptions could limit the combined company’s ability to achieve the anticipated benefits of the merger. The adverse effect of any such disruptions could also be exacerbated by a delay in the completion of the merger or by a termination of the merger agreement.

The combined company may be exposed to increased litigation, which could have an adverse effect on the combined company’s business and operations.

The combined company may be exposed to increased litigation from stockholders, customers, suppliers, consumers and other third parties due to the combination of Entegris’ business and Versum’s business following the merger. Such litigation may have an adverse impact on the combined company’s business and results of operations or may cause disruptions to the combined company’s operations.

Combining the businesses of Entegris and Versum may be more difficult, costly or time-consuming than expected and the combined company may fail to realize the anticipated benefits of the merger, which may adversely affect the combined company’s business results and negatively affect the value of the common stock of the combined company following the merger.

The success of the merger will depend on, among other things, the ability of Entegris and Versum to combine their businesses in a manner that facilitates growth opportunities and realizes cost savings. Entegris and Versum have entered into the merger agreement because each believes that the merger and the other transactions contemplated by the merger agreement are fair to and in the best interests of its respective stockholders and that combining the businesses of Entegris and Versum will produce benefits and cost savings. See also the section entitled “The Merger—Recommendation of the Entegris Board of Directors; Entegris’ Reasons for the Merger” beginning on page 87 and “The Merger—Recommendation of the Versum Board of Directors; Versum’s Reasons for the Merger” beginning on page 91.

However, Entegris and Versum must successfully combine their respective businesses in a manner that permits these benefits to be realized. In addition, the combined company must achieve the anticipated growth and cost savings without adversely affecting current revenues and investments in future growth. If the combined company is not able to successfully achieve these objectives, the anticipated benefits of the merger may not be realized fully, or at all, or may take longer to realize than expected.

An inability to realize the full extent of the anticipated benefits of the merger and the other transactions contemplated by the merger agreement, as well as any delays encountered in the integration process, could have an adverse effect upon the revenues, level of expenses and operating results of the combined company, which may adversely affect the value of the common stock of the combined company after the completion of the merger.

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In addition, the actual integration may result in additional and unforeseen expenses, and the anticipated benefits of the integration plan may not be realized. Actual growth and cost savings, if achieved, may be lower than what Entegris and Versum expect and may take longer to achieve than anticipated. If Entegris and Versum are not able to adequately address integration challenges, they may be unable to successfully integrate their operations or realize the anticipated benefits of the integration of the two companies.

The failure to successfully integrate the businesses and operations of Entegris and Versum in the expected time frame may adversely affect the combined company’s future results.

Entegris and Versum have operated and, until the completion of the merger, will continue to operate independently. There can be no assurances that their businesses can be integrated successfully. It is possible that the integration process could result in the loss of key Entegris employees or key Versum employees, the loss of customers, the disruption of either company’s or both companies’ ongoing businesses, inconsistencies in standards, controls, procedures and policies, unexpected integration issues, higher than expected integration costs and an overall post-completion integration process that takes longer than originally anticipated. Specifically, the following issues, among others, must be addressed in integrating the operations of Entegris and Versum in order to realize the anticipated benefits of the merger so the combined company performs as expected:

In addition, at times the attention of certain members of either company’s or both companies’ management and resources may be focused on completion of the merger and the integration of the businesses of the two companies and diverted from day-to-day business operations or other opportunities that may have been beneficial to such company, which may disrupt each company’s ongoing business and the business of the combined company.

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Furthermore, the board of directors and executive leadership of the combined company will consist of former directors and executive officers from each of Entegris and Versum. Combining the boards of directors and management teams of each company into a single board and a single management team could require the reconciliation of differing priorities and philosophies.

The Entegris and Versum unaudited prospective financial information is inherently subject to uncertainties, the unaudited pro forma financial data included in this document is preliminary and the combined company’s actual financial position and results of operations after the merger may differ materially from these estimates and the unaudited pro forma financial data included in this joint proxy statement/prospectus. The unaudited pro forma combined financial data does not reflect the effect of any divestitures that may be required in connection with the merger.

The unaudited pro forma combined financial statements and unaudited pro forma per share data included in this joint proxy statement/prospectus are presented for illustrative purposes only, contain a variety of adjustments, assumptions and preliminary estimates and are not necessarily indicative of what the combined company’s actual financial position or results of operations would have been had the merger been completed on the dates indicated. The combined company’s actual results and financial position after the merger may differ materially and adversely from the unaudited pro forma financial data included in this joint proxy statement/prospectus. The unaudited pro forma combined financial information does not reflect the effect of any divestitures that may be required in connection with the merger. For more information, see the sections entitled “Comparative Historical and Unaudited Pro Forma Per Share Data” beginning on page 39 and “Unaudited Pro Forma Condensed Combined Financial Statements” beginning on page 158.

While presented with numeric specificity, the Entegris and Versum unaudited prospective financial information provided in this joint proxy statement/prospectus is based on numerous variables and assumptions (including, but not limited to, those related to industry performance and competition, general business, the specialty materials and related industries, and economic, market and financial conditions and additional matters specific to Entegris’ or Versum’s business, as applicable) that are inherently subjective and uncertain and are beyond the control of the respective management teams of Entegris and Versum. As a result, actual results may differ materially from the unaudited prospective financial information. Important factors that may affect actual results and cause these unaudited projected financial forecasts to not be achieved include, but are not limited to, risks and uncertainties relating to Entegris’ or Versum’s business, as applicable (including each company’s ability to achieve strategic goals, objectives and targets over applicable periods), industry performance, general business and economic conditions. For more information see the sections entitled “The Merger—Entegris Unaudited Financial Projections” beginning on page 115, “The Merger—Versum Unaudited Financial Projections” beginning on page 118 and “The Merger—Certain Estimated Synergies” beginning on page 121.

The combined company may be unable to retain Entegris and Versum personnel successfully after the merger is completed.

The success of the merger will depend in part on the combined company’s ability to retain the talents and dedication of the professionals currently employed by Entegris and Versum. It is possible that these employees may decide not to remain with Entegris or Versum, as applicable, while the merger is pending, or with the combined company after the merger is consummated. If key employees terminate their employment, or if an insufficient number of employees are retained to maintain effective operations, the combined company’s business activities may be adversely affected and management’s attention may be diverted from successfully integrating Entegris and Versum to hiring suitable replacements, all of which may cause the combined company’s business to suffer. In addition, Entegris and Versum may not be able to locate suitable replacements for any key employees that leave either company or offer employment to potential replacements on reasonable terms.

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The combined company’s debt may limit its financial flexibility.

Entegris and Versum continue to review the treatment of their existing indebtedness. Entegris and Versum may seek to repay, refinance, repurchase, redeem, exchange or otherwise terminate their existing indebtedness prior to, in connection with or following the completion of the merger. If either Entegris or Versum seeks to refinance its existing indebtedness, there can be no guarantee that it will be able to execute the refinancing on favorable terms or at all. Alternatively, Entegris and Versum may seek to leave all or a portion of their existing indebtedness outstanding as the primary obligation of the combined company or to incur additional indebtedness or refinancing indebtedness prior to, in connection with or following the completion of the merger.

Entegris’ or Versum’s substantial indebtedness could have adverse effects on such company’s and/or the combined company’s financial condition and results of operations, including:

The companies’ ability to arrange any additional financing for the purposes described above or otherwise will depend on, among other factors, the companies’ respective financial positions and performance, as well as prevailing market conditions and other factors beyond their control. The level and quality of the combined company’s earnings, operations, business and management, among other things, will impact the determination of the combined company’s credit ratings. A decrease in the ratings assigned to the combined company by the ratings agencies may negatively impact the combined company’s access to the debt capital markets and increase the combined company’s cost of borrowing. There can be no assurance that the combined company will be able to obtain financing on acceptable terms or at all. In addition, there can be no assurance that the combined company will be able to maintain the current credit worthiness or prospective credit ratings of Entegris or Versum, and any actual or anticipated changes or downgrades in such credit ratings may have a negative impact on the liquidity, capital position or access to capital markets of the combined company.

If the existing indebtedness of Entegris and/or Versum remains outstanding, or if either company refinances its existing indebtedness, covenants contained in the agreements governing such indebtedness will impose restrictions on the combined company and certain of its subsidiaries that may affect their ability to operate their businesses.

The agreements that govern the indebtedness of Entegris and Versum, in addition to any refinanced indebtedness, may contain various affirmative and negative covenants. Such covenants may, subject to certain significant exceptions, restrict the ability of the combined company and certain of its subsidiaries to, among other things, incur liens, incur debt, engage in mergers, consolidations and acquisitions, transfer assets outside the ordinary course of business, make loans or other investments, pay dividends, repurchase equity interests, make other payments with respect to equity interests, repay or repurchase subordinated debt and engage in affiliate transactions. In addition, the agreements governing the existing indebtedness of Entegris and Versum contain financial covenants that would require the combined company to maintain certain financial ratios under certain circumstances. The ability of the combined company and its subsidiaries to comply with these provisions may be affected by events beyond their control. Failure to comply with these covenants could result in an event of default, which, if not cured or waived, could accelerate the combined company’s repayment obligations.

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Declaration, payment and amounts of dividends, if any, distributed to stockholders of the combined company will be uncertain.

Whether any dividends are declared or paid to stockholders of the combined company following the merger, and the amounts of any such dividends that are declared or paid, are uncertain and depend on a number of factors. If dividends are paid to stockholders of the combined company, they may not be of the same amount as paid by Entegris or Versum to their respective stockholders prior to the merger. The board of directors of the combined company will have the discretion to determine the dividend policy of the combined company, which may be impacted by any of the following factors:

Stockholders should be aware that they have no contractual or other legal right to dividends that have not been declared.

Risks Relating to Entegris’ Business

Entegris’ business will continue to be subject to the risks described in the sections entitled “Risk Factors” in Entegris’ Annual Report on Form 10-K for the fiscal year ended December 31, 2018, and in other documents incorporated by reference into this joint proxy statement/prospectus. See the section entitled “Where You Can Find More Information” beginning on page 213 for the location of information incorporated by reference into this joint proxy statement/prospectus.

Risks Relating to Versum’s Business

Versum’s business will continue to be subject to the risks described in the sections entitled “Risk Factors” in Versum’s Annual Report on Form 10-K for the year ended September 30, 2018, Versum’s Quarterly Report on Form 10-Q for the quarterly period ended December 31, 2018 and in other documents incorporated by reference into this joint proxy statement/prospectus. See the section entitled “Where You Can Find More Information” beginning on page 213 for the location of information incorporated by reference into this joint proxy statement/prospectus.

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THE PARTIES TO THE MERGER

Entegris, Inc.

129 Concord Road

Billerica, Massachusetts 01821

(978) 436-6500

Entegris is a leading global developer, manufacturer and supplier of microcontamination control products, specialty chemicals and advanced materials handling solutions for manufacturing processes in the semiconductor and other high-technology industries. Entegris operates in three segments: Specialty Chemicals and Engineered Materials, Microcontamination Control and Advanced Materials Handling.

Entegris common stock is listed on NASDAQ under the ticker symbol “ENTG.”

For more information about Entegris, please visit Entegris’ website at http://www.entegris.com. The information contained on Entegris’ website or accessible through it (other than the documents incorporated by reference herein) does not constitute a part of this joint proxy statement/prospectus or any other report or document on file with or furnished to the SEC. Additional information about Entegris is included in the documents incorporated by reference into this joint proxy statement/prospectus. See the section entitled “Where You Can Find More Information” beginning on page 213.

Versum Materials, Inc.

8555 South River Parkway

Tempe, Arizona 85284

(602) 282-1000

Versum is a global provider of innovative solutions to the semiconductor and display industries with expertise in the development, manufacturing, transportation and handling of specialty materials. Versum employs expertise in molecular design and synthesis, purification, advanced analytics, formulation development and containers and delivery systems for the handling of high purity materials to deliver leading-edge solutions and critical process support to Versum’s customers.

Versum common stock is listed on the NYSE under the ticker symbol “VSM.”

For more information about Versum, please visit Versum’s website at http://www.versummaterials.com. The information contained on Versum’s website or accessible through it (other than the documents incorporated by reference herein) does not constitute a part of this joint proxy statement/prospectus or any other report or document on file with or furnished to the SEC. Additional information about Versum is included in the documents incorporated by reference into this joint proxy statement/prospectus. See the section entitled “Where You Can Find More Information” beginning on page 213.

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THE ENTEGRIS SPECIAL MEETING

This joint proxy statement/prospectus is first being mailed on or about March 22, 2019 and constitutes notice of the Entegris special meeting in conformity with the requirements of the DGCL and the by-laws of Entegris, which are referred to as the Entegris by-laws.

This joint proxy statement/prospectus is being provided to Entegris stockholders as part of a solicitation of proxies by the Entegris board of directors for use at the Entegris special meeting and at any adjournments or postponements of the Entegris special meeting. Entegris stockholders are encouraged to read the entire document carefully, including the annexes to and documents incorporated by reference into this document, for more detailed information regarding the merger agreement and the transactions contemplated by the merger agreement.

Date, Time and Place of the Entegris Special Meeting

The Entegris special meeting is scheduled to be held at Entegris’ headquarters at 129 Concord Road, Billerica, Massachusetts, on April 26, 2019, beginning at 12:00 p.m., Eastern Time, unless postponed to a later date.

Matters to be Considered at the Entegris Special Meeting

The purposes of the Entegris special meeting are as follows, each as further described in this joint proxy statement/prospectus:

Recommendation of the Entegris Board of Directors

The Entegris board of directors unanimously recommends that Entegris stockholders vote:

After careful consideration, the Entegris board of directors unanimously (1) determined that the merger agreement and the transactions contemplated thereby, including but not limited to the merger, the share issuance and the adoption of the amended and restated certificate of incorporation on the terms set forth in the merger agreement are fair to, and in the best interests of, Entegris and the holders of shares of Entegris common stock; (2) approved and declared advisable the merger agreement and the transactions contemplated thereby, including the merger, on the terms and subject to the conditions set forth in the merger agreement; (3) directed that the share issuance and the merger agreement be submitted to the holders of shares of Entegris common stock for their approval and adoption, and the amended and restated certificate of incorporation be submitted to the holders of shares of Entegris common stock for their adoption; and (4) resolved to recommend that the holders of shares of Entegris common stock vote in favor of the adoption of the merger agreement, including the approval of the share issuance, and the adoption of the amended and restated certificate of incorporation.

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See also the section entitled “The Merger—Recommendation of the Entegris Board of Directors; Entegris’ Reasons for the Merger” beginning on page 87.

Record Date for the Entegris Special Meeting and Voting Rights

The record date to determine who is entitled to receive notice of and to vote at the Entegris special meeting or any adjournments or postponements thereof is April 2, 2019. As of the close of business on March 11, 2019, the latest practicable date prior to the date of this joint proxy statement/prospectus, there were 135,513,636 shares of Entegris common stock issued and outstanding, each entitled to vote at the Entegris special meeting. Each Entegris stockholder will have one vote for any matter properly brought before the Entegris special meeting for each share of Entegris common stock such holder owned at the close of business on the Entegris record date. Only Entegris stockholders of record at the close of business on the Entegris record date are entitled to receive notice of and to vote at the Entegris special meeting and any and all adjournments or postponements thereof.

Quorum; Abstentions and Broker Non-Votes

A quorum of stockholders is necessary to conduct the Entegris special meeting. The holders of a majority of the shares of Entegris common stock issued and outstanding and entitled to vote at the meeting must be represented at the Entegris special meeting in person or by proxy in order to constitute a quorum. Abstentions will be counted for purposes of determining whether a quorum exists. If a quorum is not present, the Entegris special meeting will be postponed until the holders of the number of shares of Entegris common stock required to constitute a quorum attend.

Banks, brokers or other nominees who hold shares in “street name” for a beneficial owner of those shares typically have the authority to vote in their discretion on “routine” proposals when they have not received instructions from beneficial owners. However, banks, brokers or other nominees are not allowed to exercise their voting discretion with respect to the approval of matters that the NASDAQ determines to be “non-routine.” Generally, a broker non-vote occurs on an item when a bank, broker or other nominee returns a proxy but does not provide instructions as to how shares should be voted on a particular matter. Because none of the proposals to be voted on at the Entegris special meeting are “routine” matters for which brokers may have discretionary authority to vote, Entegris does not expect any broker non-votes at the Entegris special meeting. As a result, if you hold your shares of Entegris common stock in “street name,” your shares will not be represented and will not be voted on any matter unless you affirmatively instruct your bank, broker or other nominee how to vote your shares in one of the ways indicated by your bank, broker or other nominee. It is therefore critical that you cast your vote by instructing your bank, broker or other nominee on how to vote.

If you submit a properly executed WHITE proxy card, even if you abstain from voting or vote against the adoption of the merger or the amended and restated certificate of incorporation, your shares of Entegris common stock will be counted for purposes of calculating whether a quorum is present at the Entegris special meeting. Executed but unvoted proxies will be voted in accordance with the recommendations of the Entegris board of directors. If additional votes must be solicited to adopt the merger agreement and the amended and restated certificate of incorporation, it is expected that the meeting will be adjourned to solicit additional proxies.

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Required Votes; Vote of Entegris’ Directors and Executive Officers

Except for the Entegris adjournment proposal, the vote required to approve all of the proposals listed herein assumes the presence of a quorum.

As of March 11, 2019, the latest practicable date prior to the date of this joint proxy statement/prospectus, Entegris directors and executive officers and their affiliates, as a group, owned and were entitled to vote 1,125,061 shares of Entegris common stock, or approximately 0.8% of the total outstanding shares of Entegris common stock. Although none of them has entered into any agreement obligating them to do so, Entegris currently expects that all of its directors and executive officers will vote their shares “FOR” the Entegris merger agreement proposal, “FOR” the Entegris charter proposal, “FOR” the Entegris compensation proposal and “FOR” the Entegris adjournment proposal. See also the section entitled “Interests of Entegris’ Directors and Executive Officers in the Merger” beginning on page 170 and the arrangements described in Part III of Entegris’ Annual Report on Form 10-K for the fiscal year ended on December 31, 2018 and Entegris’ Definitive Proxy Statement on Schedule 14A for Entegris’ 2019 annual meeting filed with the SEC on March 20, 2019, both of which are incorporated into this joint proxy statement/prospectus by reference.

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Methods of Voting

If you are a stockholder of record, you may vote by proxy through the Internet, by telephone or by mail, or by voting in person at the Entegris special meeting. For shares held through a bank, broker or other nominee in “street name” instead of as a registered holder, you may vote by submitting your voting instructions to your bank, broker or other nominee. In most instances, you will be able to do this over the Internet, by telephone or by mail as indicated below. Please refer to the information from your bank, broker or other nominee on how to submit voting instructions. If you do not provide voting instructions to your bank, broker or other nominee, your shares of Entegris common stock will not be voted on any proposal as your bank, broker or other nominee does not have discretionary authority to vote on any of the proposals to be voted on at the Entegris special meeting; see the section entitled “The Entegris Special Meeting—Quorum; Abstentions and Broker Non-Votes” beginning on page 60.

If you are a stockholder of record, proxies submitted over the Internet, by telephone or by mail as described above must be received by 11:59 p.m., Eastern Time, on April 25, 2019.

Notwithstanding the above, if your shares are held in “street name” by a bank, broker or other nominee, you should follow the instructions you receive from your bank, broker or other nominee on how to vote your shares. Registered stockholders who attend the Entegris special meeting may vote their shares personally even if they previously have voted their shares.

If you deliver a proxy pursuant to this joint proxy statement/prospectus, but do not specify a choice with respect to any proposal set forth in this joint proxy statement/prospectus, your underlying shares of Entegris common stock will be voted on such uninstructed proposal in accordance with the recommendation of the Entegris board of directors. Entegris does not expect that any matter other than the proposals listed above will be brought before the Entegris special meeting and Entegris by-laws provide that the only business that may be conducted at the Entegris special meeting are those proposals brought before the meeting pursuant to this joint proxy statement/prospectus.

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Revocability of Proxies

Any stockholder giving a proxy has the right to revoke it before the proxy is voted at the Entegris special meeting by any of the following actions:

Execution or revocation of a proxy will not in any way affect the stockholder’s right to attend the special meeting and vote in person.

Written notices of revocation and other communications with respect to the revocation of proxies should be addressed to:

Entegris, Inc.

Attn: Corporate Secretary

129 Concord Road

Billerica, Massachusetts 01821

If your shares are held in “street name” and you previously provided voting instructions to your broker, bank or other nominee, you should follow the instructions provided by your broker, bank or other nominee to revoke or change your voting instructions.

Proxy Solicitation Costs

Entegris is soliciting proxies to provide an opportunity to all Entegris stockholders to vote on agenda items, whether or not the stockholders are able to attend the Entegris special meeting or an adjournment or postponement thereof. Entegris will bear the entire cost of soliciting proxies from its stockholders, except that Versum and Entegris have agreed to each pay one half of the costs and expenses of filing, printing and mailing this joint proxy statement/prospectus and all filing and other similar fees payable to the SEC in connection with this joint proxy statement/prospectus. In addition to the solicitation of proxies by mail, Entegris will ask banks, brokers and other custodians, nominees and fiduciaries to forward the proxy solicitation materials to the beneficial owners of shares of Entegris common stock held of record by such nominee holders. Entegris may be required to reimburse these nominee holders for their customary clerical and mailing expenses incurred in forwarding the proxy solicitation materials to the beneficial owners.

Entegris has retained MacKenzie to assist in the solicitation process. Entegris estimates that it will pay MacKenzie a fee of approximately $25,000, plus reimbursement of reasonable expenses. Entegris also has agreed to indemnify MacKenzie against various liabilities and expenses that relate to or arise out of its solicitation of proxies (subject to certain exceptions). Proxies may be solicited on behalf of Entegris or by Entegris directors, officers and other employees in person, by mail, by telephone, by facsimile, by messenger, via the Internet or by other means of communication, including electronic communication. Directors, officers and employees of Entegris will not be paid any additional amounts for their services or solicitation in this regard.

Attending the Entegris Special Meeting

You are entitled to attend the Entegris special meeting only if you are a stockholder of record of Entegris at the close of business on April 2, 2019 (the record date for the Entegris special meeting) or you hold your shares

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of Entegris beneficially in the name of a broker, bank or other nominee as of the Entegris record date, or you hold a valid proxy for the Entegris special meeting.

If you are a stockholder of record of Entegris at the close of business on April 2, 2019 and wish to attend the Entegris special meeting, please so indicate on the appropriate WHITE proxy card or as prompted by the Internet or telephone voting system. Your name will be verified against the list of stockholders of record prior to your being admitted to the Entegris special meeting.

If a broker, bank or other nominee is the record owner of your shares of Entegris common stock, you will need to have proof that you are the beneficial owner as of the Entegris record date to be admitted to the Entegris special meeting. A recent statement or letter from your broker, bank or other nominee confirming your ownership as of the Entegris record date, or presentation of a valid proxy from a broker, bank or other nominee that is the record owner of your shares, would be acceptable proof of your beneficial ownership.

You should be prepared to present photo identification for admittance. If you do not provide photo identification or comply with the other procedures outlined above upon request, you might not be admitted to the Entegris special meeting.

Householding

SEC rules permit companies and intermediaries such as brokers to satisfy delivery requirements for proxy statements and notices with respect to two or more stockholders sharing the same address by delivering a single proxy statement or a single notice addressed to those stockholders. This process, which is commonly referred to as “householding,” provides cost savings for companies. Some brokers household proxy materials, delivering a single proxy statement or notice to multiple stockholders sharing an address unless contrary instructions have been received from the affected stockholders. Once you have received notice from your broker that they will be householding materials to your address, householding will continue until you are notified otherwise or until you revoke your consent. If, at any time, you no longer wish to participate in householding and would prefer to receive a separate proxy statement or notice, or if your household is receiving multiple copies of these documents and you wish to request that future deliveries be limited to a single copy, please notify your broker. You can request prompt delivery of a copy of this joint proxy statement/prospectus by writing to: Corporate Secretary, Entegris, Inc., 129 Concord Road, Billerica Massachusetts 01821 or by calling (978) 436-6500.

Tabulation of Votes

The Entegris board of directors will appoint an independent inspector of election for the Entegris special meeting. The inspector of election will, among other matters, determine the number of shares of Entegris common stock represented at the Entegris special meeting to confirm the existence of a quorum, determine the validity of all proxies and ballots and certify the results of voting on all proposals submitted to Entegris stockholders.

Adjournments

If a quorum is present at the Entegris special meeting but there are not sufficient votes at the time of the Entegris special meeting to approve the Entegris merger agreement proposal and the Entegris charter proposal, then Entegris stockholders may be asked to vote on the Entegris adjournment proposal.

At any subsequent reconvening of the Entegris special meeting at which a quorum is present, any business may be transacted that might have been transacted at the original meeting and all proxies will be voted in the same manner as they would have been voted at the original convening of the Entegris special meeting, except for any proxies that have been effectively revoked or withdrawn prior to the time the proxy is voted at the reconvened meeting.

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Assistance

If you need assistance voting or in completing your WHITE proxy card or have questions regarding the Entegris special meeting, please contact MacKenzie, the proxy solicitation agent for Entegris:

MacKenzie Partners, Inc.

1407 Broadway

New York, New York 10018

Stockholders, banks and brokers call: (800) 322-2885

ENTEGRIS STOCKHOLDERS SHOULD CAREFULLY READ THIS JOINT PROXY

STATEMENT/PROSPECTUS IN ITS ENTIRETY FOR MORE DETAILED INFORMATION CONCERNING THE MERGER AGREEMENT AND THE MERGER. IN PARTICULAR, ENTEGRIS STOCKHOLDERS ARE DIRECTED TO THE MERGER AGREEMENT, WHICH IS ATTACHED AS ANNEX A HERETO.

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ENTEGRIS PROPOSAL 1: ADOPTION OF THE MERGER AGREEMENT

This joint proxy statement/prospectus is being furnished to you as a stockholder of Entegris as part of the solicitation of proxies by the Entegris board of directors for use at the Entegris special meeting to consider and vote upon a proposal to adopt the merger agreement and approve the transactions contemplated thereby, including the merger and the issuance of shares of Entegris common stock in the merger pursuant to the merger agreement, which is attached as Annex A to this joint proxy statement/prospectus.

The Entegris board of directors, after due and careful discussion and consideration, unanimously approved and declared advisable the merger agreement, the merger and the other transactions contemplated by the merger agreement and determined that the merger agreement, the merger and the other transactions contemplated by the merger agreement are fair to and in the best interests of Entegris and its stockholders.

The Entegris board of directors accordingly unanimously recommends that Entegris stockholders adopt the merger agreement, as disclosed in this joint proxy statement/prospectus and particularly the related narrative disclosures in the sections of this joint proxy statement/prospectus entitled “The Merger” beginning on page 80 and “The Merger Agreement” beginning on page 127 and as attached as Annex A to this joint proxy statement/prospectus.

The merger between Entegris and Versum cannot be completed without the affirmative vote of a majority of the outstanding shares of Entegris common stock entitled to vote thereon. A failure to vote, a broker non-vote or an abstention will have the same effect as a vote “AGAINST” the proposal to adopt the merger agreement.

IF YOU ARE AN ENTEGRIS STOCKHOLDER, THE ENTEGRIS BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT YOU VOTE “FOR” THE ENTEGRIS MERGER AGREEMENT PROPOSAL (ENTEGRIS PROPOSAL 1)

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ENTEGRIS PROPOSAL 2: ADOPTION OF THE AMENDED AND RESTATED CERTIFICATE OF INCORPORATION OF ENTEGRIS

The Entegris board of directors has approved and declared advisable, pursuant to the merger agreement, the adoption of the amended and restated certificate of incorporation. The amended and restated certificate of incorporation is attached as Annex B to this joint proxy statement/prospectus. Entegris stockholders should read the amended and restated certificate of incorporation in its entirety.

The key amendments included in the amended and restated certificate of incorporation are as follows:

In addition to the key provisions discussed above, in an attempt to streamline the combined company charter, the Entegris certificate of incorporation will be restated.

The approval of Entegris stockholders to amend and restate the certificate of incorporation is not required in order to complete the merger. If this proposal to adopt the amended and restated certificate of incorporation is approved but the Entegris merger agreement proposal is not approved, the Entegris board of directors will abandon the amended and restated certificate of incorporation without further action by Entegris stockholders.

The Entegris board of directors unanimously recommends that Entegris stockholders adopt the amended and restated certificate of incorporation described herein and set forth in full as Annex B.

Approval of the Entegris charter proposal requires the affirmative vote of a majority of the outstanding shares of Entegris common stock entitled to vote on the proposal. A failure to vote, a broker non-vote or an abstention will have the same effect as a vote “AGAINST” the Entegris charter proposal.

IF YOU ARE AN ENTEGRIS STOCKHOLDER, THE ENTEGRIS BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT YOU VOTE “FOR” THE ENTEGRIS CHARTER PROPOSAL (ENTEGRIS PROPOSAL 2)

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ENTEGRIS PROPOSAL 3: ADVISORY (NON-BINDING) VOTE ON MERGER-RELATED COMPENSATION FOR NAMED EXECUTIVE OFFICERS

Pursuant to Section 14A of the Exchange Act and Rule 14a-21(c) thereunder, Entegris is seeking a non-binding, advisory stockholder approval of the compensation of Entegris’ named executive officers that is based on or otherwise relates to the merger as disclosed in the section entitled “Interests of Entegris’ Directors and Executive Officers in the Merger—Quantification of Payments and Benefits to Entegris’ Named Executive Officers” beginning on page 173. The proposal gives Entegris stockholders the opportunity to express their views on the merger-related compensation of Entegris’ named executive officers.

Accordingly, Entegris is asking Entegris stockholders to vote “FOR” the adoption of the following resolution, on a non-binding, advisory basis:

“RESOLVED, that the compensation that will or may be paid or become payable to Entegris’ named executive officers, in connection with the merger, and the agreements or understandings pursuant to which such compensation will or may be paid or become payable, in each case as disclosed pursuant to Item 402(t) of Regulation S-K in “Interests of Entegris’ Directors and Executive Officers in the Merger—Quantification of Payments and Benefits to Entegris’ Named Executive Officers” are hereby APPROVED.”

The vote on the advisory compensation proposal is a vote separate and apart from the vote on the proposals to adopt the merger agreement and the amended and restated certificate of incorporation. Accordingly, if you are an Entegris stockholder, you may vote to adopt the Entegris merger agreement proposal and/or the Entegris charter proposal, and vote not to approve the Entegris compensation proposal, and vice versa. If the merger is completed, the merger-related compensation may be paid to Entegris’ named executive officers to the extent payable in accordance with the terms of the compensation agreements and arrangements even if Entegris stockholders fail to approve the advisory vote regarding merger-related compensation.

Assuming a quorum is present at the Entegris special meeting, approval of the Entegris compensation proposal requires the affirmative vote of a majority of votes properly cast on the proposal at the Entegris special meeting. An abstention, a broker non-vote or other failure to vote (including a failure to instruct your bank, broker or other nominee to vote) will have no effect on the Entegris compensation proposal, assuming a quorum is present.

The Entegris board of directors unanimously recommends a vote “FOR” the advisory compensation proposal.

IF YOU ARE AN ENTEGRIS STOCKHOLDER, THE ENTEGRIS BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT YOU VOTE “FOR” THE ENTEGRIS COMPENSATION PROPOSAL (ENTEGRIS PROPOSAL 3)

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ENTEGRIS PROPOSAL 4: ADJOURNMENT OF THE ENTEGRIS SPECIAL MEETING

The Entegris special meeting may be adjourned to another time and place if necessary to permit solicitation of additional proxies if there are not sufficient votes to approve the Entegris merger agreement proposal and the Entegris charter proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to Entegris stockholders.

Entegris is asking its stockholders to authorize the holder of any proxy solicited by the Entegris board of directors to vote in favor of any adjournment to the Entegris special meeting to solicit additional proxies if there are not sufficient votes to approve the Entegris merger agreement proposal and the Entegris charter proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to Entegris stockholders.

The Entegris board of directors unanimously recommends that Entegris stockholders approve the proposal to adjourn the Entegris special meeting, if necessary.

Whether or not there is a quorum, approval of the Entegris adjournment proposal requires the approval of the holders of a majority of the voting shares represented at the Entegris special meeting. A stockholder’s abstention from voting will have the same effect as a vote “AGAINST” the Entegris adjournment proposal, while a broker non-vote or other failure to vote (including a failure to instruct your bank, broker or other nominee to vote) will have no effect on the outcome of the Entegris adjournment proposal.

IF YOU ARE AN ENTEGRIS STOCKHOLDER, THE ENTEGRIS BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT YOU VOTE “FOR” THE ENTEGRIS ADJOURNMENT PROPOSAL (ENTEGRIS PROPOSAL 4)

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THE VERSUM SPECIAL MEETING

This joint proxy statement/prospectus is first being mailed on or about March 22, 2019 and constitutes notice of the Versum special meeting in conformity with the requirements of the DGCL and the amended and restated bylaws of Versum, which are referred to as the Versum bylaws.

This joint proxy statement/prospectus is being provided to Versum stockholders as part of a solicitation of proxies by the Versum board of directors for use at the Versum special meeting and at any adjournments or postponements of the Versum special meeting. Versum stockholders are encouraged to read the entire document carefully, including the annexes to and documents incorporated by reference into this document, for more detailed information regarding the merger agreement and the transactions contemplated by the merger agreement.

Date, Time and Place of the Versum Special Meeting

The Versum special meeting is scheduled to be held at 8555 South River Parkway, Tempe, Arizona 85284, on April  26, 2019, beginning at 9:00 a.m., Mountain Standard Time, unless postponed to a later date.

Matters to Be Considered at the Versum Special Meeting

The purposes of the Versum special meeting are as follows, each as further described in this joint proxy statement/prospectus:

Recommendation of the Versum Board of Directors

The Versum board of directors unanimously recommends that Versum stockholders vote:

After careful consideration, the Versum board of directors unanimously (1) determined that the merger agreement, the merger and the other transactions contemplated by the merger agreement are fair to, and in the best interests of, Versum and its stockholders; (2) approved and declared advisable the merger agreement, the merger and the other transactions contemplated by the merger agreement; (3) directed that the merger agreement be submitted for adoption at a meeting of Versum stockholders; and (4) resolved to recommend that Versum stockholders vote in favor of the adoption of the merger agreement.

See also the section entitled “The Merger—Recommendation of the Versum Board of Directors; Versum’s Reasons for the Merger” beginning on page 91.

Record Date for the Versum Special Meeting and Voting Rights

The record date to determine who is entitled to receive notice of and to vote at the Versum special meeting or any adjournments or postponements thereof is April 2, 2019. As of the close of business on March 11, 2019,

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the latest practicable date prior to the date of this joint proxy statement/prospectus, there were 109,143,954 shares of Versum common stock issued and outstanding and entitled to vote at the Versum special meeting. Each Versum stockholder is entitled to one vote for any matter properly brought before the Versum special meeting for each share of Versum common stock such holder owned at the close of business on the Versum record date. Only Versum stockholders of record at the close of business on the Versum record date are entitled to receive notice of and to vote at the Versum special meeting and any and all adjournments or postponements thereof.

Quorum; Abstentions and Broker Non-Votes

A quorum of stockholders is necessary to conduct the Versum special meeting. The presence, in person or by proxy, of the holders of a majority of the shares of Versum common stock entitled to vote at the Versum special meeting is necessary to constitute a quorum. Shares of Versum common stock represented at the Versum special meeting and entitled to vote, but not voted, including shares for which a stockholder directs an “abstention” from voting and broker non-votes, will be counted for purposes of determining a quorum. If a quorum is not present, the Versum special meeting will be postponed until the holders of the number of shares of Versum common stock required to constitute a quorum attend.

Under the NYSE rules, banks, brokers or other nominees who hold shares in “street name” for a beneficial owner of those shares typically have the authority to vote in their discretion on “routine” proposals when they have not received instructions from beneficial owners. However, banks, brokers or other nominees are not allowed to exercise their voting discretion with respect to the approval of matters that the NYSE determines to be “non-routine.” Generally, a broker non-vote occurs on an item when (a) a bank, broker or other nominee has discretionary authority to vote on one or more “routine” proposals to be voted on at a meeting of stockholders, but is not permitted to vote on other “non-routine” proposals without instructions from the beneficial owner of the shares and (b) the beneficial owner fails to provide the bank, broker or other nominee with such instructions. Under the NYSE rules, “non-routine” matters include the Versum merger agreement proposal (Versum Proposal 1), the Versum compensation proposal (Versum Proposal 2) and the Versum adjournment proposal (Versum Proposal 3). Because none of the proposals to be voted on at the Versum special meeting are routine matters for which brokers may have discretionary authority to vote, Versum does not expect any broker non-votes at the Versum special meeting. As a result, if you hold your shares of Versum common stock in “street name,” your shares will not be represented and will not be voted on any matter unless you affirmatively instruct your bank, broker or other nominee how to vote your shares in one of the ways indicated by your bank, broker or other nominee. It is therefore critical that you cast your vote by instructing your bank, broker or other nominee on how to vote. The NYSE rules governing brokers’ discretionary authority will not permit brokers to exercise discretionary authority regarding any of the proposals to be voted on at the Versum special meeting.

Required Votes; Vote of Versum’s Directors and Executive Officers

Except for the Versum adjournment proposal, the vote required to approve all of the proposals listed herein assumes the presence of a quorum.

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As of March 11, 2019, the latest practicable date prior to the date of this joint proxy statement/prospectus, Versum directors and executive officers, and their affiliates, as a group, owned and were entitled to vote 679,695 shares of Versum common stock, or approximately 0.62% of the total outstanding shares of Versum common stock. Although none of them has entered into any agreement obligating them to do so, Versum currently expects that all of its directors and executive officers will vote their shares “FOR” the Versum merger agreement proposal, “FOR” the Versum compensation proposal and “FOR” the Versum adjournment proposal. See also the section entitled “Interests of Versum’s Directors And Executive Officers In The Merger” beginning on page 176 and the arrangements described in Part III of Versum’s Annual Report on Form 10-K for the year ended September 30, 2018 and Versum’s Definitive Proxy Statement on Schedule 14A for Versum’s annual meeting filed with the SEC on December 20, 2018, both of which are incorporated into this joint proxy statement/prospectus by reference.

Methods of Voting

If you are a stockholder of record, you may vote by proxy through the Internet, by telephone or by mail, or by voting in person at the Versum special meeting. For shares held through a bank, broker or other nominee in “street name” instead of as a registered holder, you may vote by submitting your voting instructions to your bank, broker or other nominee. In most instances, you will be able to do this over the Internet, by telephone or by mail as indicated below. Please refer to the information from your bank, broker or other nominee on how to submit voting instructions. If you do not provide voting instructions to your bank, broker or other nominee, your shares of Versum common stock will not be voted on any proposal as your bank, broker or other nominee does not have discretionary authority to vote on any of the proposals to be voted on at the Versum special meeting; see the section entitled “The Versum Special Meeting—Quorum; Abstentions and Broker Non-Votes” beginning on page 71.

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Unless revoked, all proxies representing shares entitled to vote that are delivered pursuant to this solicitation will be voted at the Versum special meeting and, where a choice has been specified on the WHITE proxy card, will be voted in accordance with such specification. To reduce administrative costs and help the environment by conserving natural resources, Versum asks that you vote through the Internet or by telephone, both of which are available 24 hours a day.

Notwithstanding the above, if you hold your shares in “street name” and you submit voting instructions to your bank, broker or other nominee, your instructions must be received by the bank, broker or other nominee prior to the deadline set forth in the information from your bank, broker or other nominee on how to submit voting instructions.

If you deliver a proxy pursuant to this joint proxy statement/prospectus, but do not specify a choice with respect to any proposal set forth in this joint proxy statement/prospectus, your underlying shares of Versum common stock will be voted on such uninstructed proposal in accordance with the recommendation of the Versum board of directors. Versum does not expect that any matter other than the proposals listed above will be brought before the Versum special meeting and the Versum bylaws provide that the only business that may be conducted at the Versum special meeting are those proposals brought before the meeting pursuant to this joint proxy statement/prospectus.

Revocability of Proxies

Any stockholder giving a proxy has the right to revoke it, including any green proxy card you may have previously submitted, before the proxy is voted at the Versum special meeting by any of the following actions:

Only your last submitted proxy card will be considered. You do not need to contact Versum to revoke any previously granted proxy you may have given by submitting a green proxy card, your submission of your vote via the instructions on your WHITE proxy card is sufficient to revoke your green proxy card.

Execution or revocation of a proxy will not in any way affect the stockholder’s right to attend the Versum special meeting and vote in person.

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Written notices of revocation and other communications with respect to the revocation of proxies should be addressed to:

Versum Materials, Inc.

Attn: Corporate Secretary

8555 South River Parkway

Tempe, Arizona 85284

If your shares are held in “street name” and you previously provided voting instructions to your broker, bank or other nominee, you should follow the instructions provided by your broker, bank or other nominee to revoke or change your voting instructions.

We urge you to discard any green proxy cards and disregard any related solicitation materials, which may have been sent to you by Merck, who is soliciting proxies from Versum stockholders in opposition to the merger. If you previously submitted a green proxy card, we urge you to cast your vote as instructed on your WHITE proxy card, which will revoke any earlier dated proxy card that you may have submitted, including any green proxy card. Only the latest dated proxy you submit will be counted.

Proxy Solicitation Costs

Versum is soliciting proxies to provide an opportunity to all Versum stockholders to vote on agenda items, whether or not the stockholders are able to attend the Versum special meeting or an adjournment or postponement thereof. Versum will bear the entire cost of soliciting proxies from its stockholders, except that Versum and Entegris have agreed to each pay one half of the costs and expenses of filing, printing and mailing this joint proxy statement/prospectus and all filing and other similar fees payable to the SEC in connection with this joint proxy statement/prospectus. In addition to the solicitation of proxies by mail, Versum will request that banks, brokers and other nominee record holders send proxies and proxy material to the beneficial owners of Versum common stock and secure their voting instructions, if necessary. Versum may be required to reimburse those banks, brokers and other nominees on request for their reasonable expenses in taking those actions.

Versum has also retained Innisfree to assist in soliciting proxies and in communicating with Versum stockholders and estimates that it will pay them a fee of approximately $750,000 plus reimbursement for certain out-of-pocket fees and expenses. Versum also has agreed to indemnify Innisfree against various liabilities and expenses that relate to or arise out of its solicitation of proxies (subject to certain exceptions). Proxies may be solicited on behalf of Versum or by Versum directors, officers and other employees in person, by mail, by telephone, by facsimile, by messenger, via the Internet or by other means of communication, including electronic communication. Directors, officers and employees of Versum will not be paid any additional amounts for their services or solicitation in this regard.

Attending the Versum Special Meeting

If you wish to attend the Versum special meeting, you must be a stockholder of record of Versum at the close of business on April 2, 2019 (the record date for the Versum special meeting), hold your shares of Versum beneficially in the name of a broker, bank or other nominee as of the Versum record date or hold a valid proxy for the Versum special meeting. If you hold your shares through a broker, bank or other nominee in “street name” (instead of as a registered holder) and you wish to vote in person at the Versum special meeting, you must obtain a legal proxy from your bank, broker or other nominee and bring the legal proxy to the meeting in order to vote in person at the Versum special meeting.

You will need to bring identification along with either your notice of special meeting or proof of stock ownership to enter the Versum special meeting. If your Versum shares are beneficially held in the name of a broker, bank or other nominee and you wish to be admitted to attend the Versum special meeting, you must

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present proof of your ownership of Versum shares, such as a bank or brokerage statement. The use of video, still photography or audio recording at the Versum special meeting is not permitted. For the safety of attendees, all bags, packages and briefcases are subject to inspection. If you do not provide photo identification or comply with the other procedures outlined above, you might not be admitted to the Versum special meeting.

If you plan to attend the Versum special meeting and vote in person, Versum still encourages you to vote in advance by the Internet, telephone or (if you received a paper copy of the proxy materials) by mail so that your vote will be counted even if you later decide not to attend the Versum special meeting. Voting your proxy by the Internet, telephone or mail will not limit your right to vote at the Versum special meeting if you later decide to attend in person.

Householding

SEC rules permit companies and intermediaries such as brokers to satisfy delivery requirements for proxy statements and notices with respect to two or more stockholders sharing the same address by delivering a single proxy statement or a single notice addressed to those stockholders. This process, which is commonly referred to as “householding,” provides cost savings for companies. Some brokers household proxy materials, delivering a single proxy statement or notice to multiple stockholders sharing an address unless contrary instructions have been received from the affected stockholders. Once you have received notice from your broker that they will be householding materials to your address, householding will continue until you are notified otherwise or until you revoke your consent. If, at any time, you no longer wish to participate in householding and would prefer to receive a separate proxy statement or notice, or if your household is receiving multiple copies of these documents and you wish to request that future deliveries be limited to a single copy, please notify your broker. You can request prompt delivery of a copy of this joint proxy statement/prospectus by writing to: Corporate Secretary, Versum Materials, Inc., 8555 South River Parkway, Tempe, Arizona 85284 or by calling (602) 282-1000.

Tabulation of Votes

The Versum board of directors will appoint an independent inspector of election for the Versum special meeting. The inspector of election will, among other matters, determine the number of shares of Versum common stock represented at the Versum special meeting to confirm the existence of a quorum, determine the validity of all proxies and ballots and certify the results of voting on all proposals submitted to Versum stockholders.

Adjournments

If a quorum is present at the Versum special meeting but there are not sufficient votes at the time of the Versum special meeting to approve the Versum merger agreement proposal, then Versum stockholders may be asked to vote on the Versum adjournment proposal.

At any subsequent reconvening of the Versum special meeting at which a quorum is present, any business may be transacted that might have been transacted at the original meeting and all proxies will be voted in the same manner as they would have been voted at the original convening of the Versum special meeting, except for any proxies that have been effectively revoked or withdrawn prior to the time the proxy is voted at the reconvened meeting.

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Assistance

If you need assistance voting or in completing your proxy card or have questions regarding the Versum special meeting, please contact Innisfree, the proxy solicitation agent for Versum:

Innisfree M&A Incorporated

501 Madison Avenue, 20th Floor

New York, New York 10022

Stockholders may call toll free: (877) 456-3463

Banks and brokers may call collect: (212) 750-5833

VERSUM STOCKHOLDERS SHOULD CAREFULLY READ THIS JOINT PROXY STATEMENT/PROSPECTUS IN ITS ENTIRETY FOR MORE DETAILED INFORMATION CONCERNING THE MERGER AGREEMENT AND THE MERGER. IN PARTICULAR, VERSUM STOCKHOLDERS ARE DIRECTED TO THE MERGER AGREEMENT, WHICH IS ATTACHED AS ANNEX A HERETO.

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VERSUM PROPOSAL 1: ADOPTION OF THE MERGER AGREEMENT

This joint proxy statement/prospectus is being furnished to you as a stockholder of Versum as part of the solicitation of proxies by the Versum board of directors for use at the Versum special meeting to consider and vote upon a proposal to adopt the merger agreement and approve the transactions contemplated thereby, including the merger, which is attached as Annex A to this joint proxy statement/prospectus.

The Versum board of directors, after due and careful discussion and consideration, unanimously approved and declared advisable the merger agreement, the merger and the other transactions contemplated by the merger agreement and determined that the merger agreement, the merger and the other transactions contemplated by the merger agreement are fair to and in the best interests of Versum and its stockholders.

The Versum board of directors accordingly unanimously recommends that Versum stockholders adopt the merger agreement, as disclosed in this joint proxy statement/prospectus and particularly the related narrative disclosures in the sections of this joint proxy statement/prospectus entitled “The Merger” beginning on page 80 and “The Merger Agreement” beginning on page 127 and as attached as Annex A to this joint proxy statement/prospectus.

The merger between Versum and Entegris cannot be completed without the affirmative vote of a majority of the outstanding shares of Versum common stock entitled to vote thereon. A failure to vote, a broker non-vote or an abstention will have the same effect as a vote “AGAINST” the proposal to adopt the merger agreement.

IF YOU ARE A VERSUM STOCKHOLDER, THE VERSUM BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT YOU VOTE “FOR” THE VERSUM MERGER AGREEMENT PROPOSAL (VERSUM PROPOSAL 1)

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VERSUM PROPOSAL 2: ADVISORY (NON-BINDING) VOTE ON MERGER-RELATED

COMPENSATION FOR NAMED EXECUTIVE OFFICERS

Pursuant to Section 14A of the Exchange Act and Rule 14a-21(c) thereunder, Versum is seeking a non-binding, advisory stockholder approval of the compensation of Versum’s named executive officers that is based on or otherwise relates to the merger as disclosed in the section entitled “Interests of Versum’s Directors and Executive Officers in the Merger—Quantification of Payments and Benefits to Versum’s Named Executive Officers—Golden Parachute Compensation” beginning on page 179. The Versum compensation proposal gives Versum stockholders the opportunity to express their views on the merger-related compensation of Versum’s named executive officers.

Accordingly, Versum is asking Versum stockholders to vote “FOR” the adoption of the following resolution, on a non-binding, advisory basis:

“RESOLVED, that the compensation that will or may be paid or become payable to Versum’s named executive officers, in connection with the merger, and the agreements or understandings pursuant to which such compensation will or may be paid or become payable, in each case as disclosed pursuant to Item 402(t) of Regulation S-K in “Interests of Versum’s Directors and Executive Officers in the Merger—Quantification of Payments and Benefits to Versum’s Named Executive Officers—Golden Parachute Compensation” are hereby APPROVED.”

The vote on the advisory compensation proposal is a vote separate and apart from the vote to adopt the merger agreement. Accordingly, if you are a Versum stockholder, you may vote to approve the Versum merger agreement proposal, and vote not to approve the Versum compensation proposal, and vice versa. If the merger is completed, the merger-related compensation may be paid to Versum’s named executive officers to the extent payable in accordance with the terms of the compensation agreements and arrangements even if Versum stockholders fail to approve the advisory vote regarding merger-related compensation.

The affirmative vote of a majority of the votes cast at the Versum special meeting on the Versum compensation proposal is required to approve the Versum compensation proposal (meaning the number of votes cast at the Versum special meeting “FOR” the Versum compensation proposal must exceed votes cast “AGAINST” in order for the Versum compensation proposal to be approved). A failure to vote, a broker non-vote or an abstention will have no effect on the outcome of the Versum compensation proposal.

The Versum board of directors unanimously recommends a vote “FOR” the advisory compensation proposal.

IF YOU ARE A VERSUM STOCKHOLDER, THE VERSUM BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT YOU VOTE “FOR” THE VERSUM COMPENSATION PROPOSAL

(VERSUM PROPOSAL 2)

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VERSUM PROPOSAL 3: ADJOURNMENT OF THE VERSUM SPECIAL MEETING

The Versum special meeting may be adjourned to another time and place if necessary to permit solicitation of additional proxies if there are not sufficient votes to approve the Versum merger agreement proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to Versum stockholders.

Versum is asking its stockholders to authorize the holder of any proxy solicited by the Versum board of directors to vote in favor of any adjournment of the Versum special meeting to solicit additional proxies if there are not sufficient votes to approve the Versum merger agreement proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to Versum stockholders.

The Versum board of directors unanimously recommends that Versum stockholders approve the proposal to adjourn the Versum special meeting, if necessary.

Whether or not a quorum is present, the affirmative vote of a majority of the votes entitled to be cast who are present in person or represented by proxy at the Versum special meeting on the Versum adjournment proposal is required to approve the Versum adjournment proposal. An abstention will have the same effect as a vote “AGAINST” the Versum adjournment proposal, while a broker non-vote or other failure to vote will have no effect on the outcome of the Versum adjournment proposal.

Under the Versum bylaws, the chairman of the Versum special meeting may adjourn the Versum special meeting regardless of the outcome of the Versum adjournment proposal.

IF YOU ARE A VERSUM STOCKHOLDER, THE VERSUM BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT YOU VOTE “FOR” THE VERSUM ADJOURNMENT PROPOSAL

(VERSUM PROPOSAL 3)

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THE MERGER

The following is a description of material aspects of the merger. While Entegris and Versum believe that the following description covers the material terms of the merger, the description may not contain all of the information that is important to you. You are encouraged to read carefully this entire joint proxy statement/prospectus, including the text of the merger agreement attached to this joint proxy statement/prospectus as Annex A, for a more complete understanding of the merger. In addition, important business and financial information about each of Entegris and Versum is included in or incorporated by reference into this joint proxy statement/prospectus. See “Where You Can Find More Information” beginning on page 213.

General

Entegris and Versum have entered into the merger agreement, which provides for the merger of Versum with and into Entegris. As a result of the merger, the separate existence of Versum will cease and Entegris will continue its existence under the laws of the State of Delaware as the surviving corporation. The combined company will be named Entegris, Inc.

Exchange Ratio

At the effective time, each share of Versum common stock (other than Versum excluded shares) will be converted into the right to receive 1.120 shares of Entegris common stock.

No fractional shares of Entegris common stock will be issued upon the conversion of shares of Versum common stock pursuant to the merger agreement. All fractional shares of Entegris common stock that a holder of shares of Versum common stock would be otherwise entitled to receive pursuant to the merger agreement will be aggregated, and such holder will be entitled to receive a cash payment, without interest, in lieu of any such fractional share, equal to the product (rounded to the nearest whole cent) of (a) the amount of such fractional share interest in a share of Entegris common stock to which such holder would be entitled pursuant to the merger agreement and (b) an amount equal to the average of the daily volume weighted average price per share of Entegris common stock on NASDAQ calculated for the ten consecutive trading days ending on the second full trading day immediately prior to (and not including) the closing date.

The exchange ratio is fixed, which means that it will not change between now and the date of the merger, regardless of whether the market price of either Entegris common stock or Versum common stock changes. Therefore, the value of the merger consideration will depend on the market price of Entegris common stock at the effective time. The market price of Entegris common stock has fluctuated since the date of the announcement of the merger agreement and will continue to fluctuate from the date of this joint proxy statement/prospectus to the date of the special meetings, the date the merger is completed and thereafter. The market price of Entegris common stock, when received by Versum stockholders after the merger is completed, could be greater than, less than or the same as the market price of Entegris common stock on the date of this joint proxy statement/prospectus or at the time of the special meeting. Accordingly, you should obtain current market quotations for Entegris common stock and Versum common stock before deciding how to vote with respect to any of the proposals described in this joint proxy statement/prospectus. Entegris common stock is traded on NASDAQ under the symbol “ENTG” and Versum common stock is traded on the NYSE under the symbol “VSM.”

Background of the Merger

The Versum board of directors and Versum senior management regularly review and assess Versum’s operations and financial performance, industry conditions and related developments as they may impact Versum’s long-term strategic plans and objectives. As part of this ongoing evaluation, following Versum’s spin-off from Air Products in 2016, the Versum board of directors, together with Versum’s senior management team, has from time to time considered various potential financial and strategic opportunities to enhance

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stockholder value, including the possibility of a combination with Entegris. Versum’s senior management and the Versum board of directors were therefore generally familiar with Entegris, its business and its management team, and believed that a potential combination of the two companies on appropriate terms and at the right time could be an opportunity to enhance stockholder value.

In furtherance of Versum’s consideration of its strategic direction, Versum has periodically consulted with Lazard over the years, including without any formal engagements, for a number of reasons that include Lazard’s experience and expertise as a financial advisor in a wide variety of transactions and its familiarity with Versum’s business.

The Entegris board of directors regularly evaluates the strategic direction of Entegris with a view towards strengthening Entegris’ business and enhancing stockholder value. As part of that evaluation, Entegris has considered, from time to time, various potential strategic transactions, including potential strategic alliances and other commercial arrangements, strategic mergers, acquisitions and divestitures, and other business combinations, including, prior to and following Versum’s spin off from Air Products in 2016, the possibility of a combination with Versum. As a result, the Entegris senior management and the Entegris board of directors were generally familiar with Versum, its management and its businesses, and believed that a combination of the two companies could be a value enhancing opportunity for the stockholders of both companies at the right time and on appropriate terms.

On December 8, 2018, Guillermo Novo, Versum’s president and chief executive officer, and Bertrand Loy, Entegris’ president and chief executive officer, met, at Mr. Novo’s invitation, in Tokyo, Japan. During this meeting, Mr. Novo raised the possibility of an all-stock merger of equals between Versum and Entegris. The two executives discussed generally the complementary nature and size of the two companies and some of the potential benefits and challenges that could result from a combination. No specific transaction terms were proposed at this meeting, but the parties discussed that the terms of the transaction and the governance of the combined company should generally be consistent with a merger of equals.

Following the December 8, 2018 meeting, Messrs. Novo and Loy spoke telephonically on two occasions to discuss the logistics of potential subsequent meetings.

On December 18, 2018, the Entegris board of directors met telephonically, together with representatives of Morgan Stanley, Entegris’ financial advisor, to discuss the potential transaction with Versum, and the potential benefits and challenges of such a transaction. The Entegris board of directors authorized Mr. Loy to continue to engage with Versum to determine if a transaction could be achieved on acceptable terms.

On December 20, 2018, Mr. Loy contacted Mr. Novo to express Entegris’ interest in continuing to discuss a potential merger transaction. Mr. Loy and Mr. Novo discussed the signing of a confidentiality agreement and beginning mutual due diligence and confirmed a meeting including Mr. Novo and Seifi Ghasemi, the chairman of the Versum board of directors, and Mr. Loy and Paul Olson, the chairman of the Entegris board of directors, in New York City on January 4, 2019.

On January 3, 2019, the parties entered into a mutual non-disclosure and standstill agreement allowing for confidential negotiations and due diligence investigations.

On January 4, 2019, Messrs. Novo, Ghasemi, Loy and Olson met in New York City to continue to discuss a potential merger. At the meeting, the parties discussed Versum’s and Entegris’ respective business philosophies, the parties’ mutual desire for an all-stock merger, potential transaction structures and potential governance and management of the combined company that would result from a merger. During this meeting, Messrs. Loy and Olson proposed that Mr. Ghasemi would serve as the chairman of the board of the combined company, that Mr. Loy would serve as the chief executive officer of the combined company and that Gregory Graves, Entegris’ chief financial officer, would serve as the chief financial officer of the combined company. Both parties indicated

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a belief that the combined company would benefit from significant potential synergies, increased scale and a strong balance sheet, and a strengthened strategic position. The parties agreed, pending authorization from the Versum board of directors, to commence due diligence with mutual management presentations to take place on January 9, 2019, with a potential second meeting among Messrs. Novo, Ghasemi, Loy and Olson to take place on January 17, 2019 in Scottsdale, Arizona, at which the parties would discuss key proposed terms for the potential transaction, including the proposed exchange ratio.

On January 5, 2019, Messrs. Novo and Loy traveled together to attend a conference in northern California. During this time, Mr. Novo and Mr. Loy discussed their respective management and organizational philosophies and business models, and identified areas in which the combined company could grow and reach new customers and also compared the cultures of the respective companies. No specific transaction terms were discussed during these conversations.

On January 7, 2019, the Versum board of directors met via a telephonic meeting, at which representatives of Versum’s senior management, Lazard and Simpson Thacher & Bartlett LLP, Versum’s legal counsel, which is referred to as Simpson Thacher, were present. Mr. Novo updated the Versum board of directors on the recent discussions with representatives of Entegris, including the January 4 meeting among Messrs. Novo, Ghasemi, Loy and Olson and the fact that Entegris and Versum had entered into a mutual non-disclosure and standstill agreement in advance of such January 4 meeting. Representatives of Lazard reviewed with the Versum board of directors Lazard’s preliminary financial view regarding a potential transaction with Entegris, and the Versum board of directors discussed the possible risks, benefits and potential terms of a potential transaction with representatives of Versum’s senior management, Lazard and Simpson Thacher. A representative of Simpson Thacher then reviewed with the Versum board of directors their fiduciary duties under applicable law. The Versum board of directors authorized and instructed Messrs. Ghasemi and Novo to proceed with the proposed subsequent meeting with representatives of Entegris, and instructed Versum’s senior management, Lazard and Simpson Thacher to proceed with due diligence.

On January 9, 2019, Mr. Novo, George Bitto, Versum’s chief financial officer, and other members of Versum’s senior management team met in Menlo Park, California with Messrs. Loy and Graves, and other members of Entegris’ senior management team, to discuss Versum’s and Entegris’ respective businesses, the potential synergies that might be achieved in the proposed combination and related matters. Representatives of each party’s financial advisor were in attendance. No specific transaction terms were discussed at this meeting. From January 9 through January 27, 2019, the parties engaged in mutual due diligence investigations of one another.

On January 12, 2019, at a telephonic meeting of the Versum board of directors, at which representatives of Versum’s senior management, Lazard and Simpson Thacher were present, Mr. Novo updated the Versum board of directors on further developments with respect to the discussions with Entegris, including that the parties had discussed an exchange of their respective management team’s financial projections. The Versum board of directors then discussed potential terms of a potential transaction with Entegris, including preliminary views on trading multiples, the potential exchange ratio and certain social issues. The Versum board of directors subsequently authorized Messrs. Novo and Ghasemi to negotiate the potential exchange ratio with representatives of Entegris based on historical trading multiple averages. Representatives of Versum’s senior management then reviewed with the Versum board of directors Versum management’s view on Versum’s financial projections, including macro forecasts, growth drivers and other assumptions, and including an update on first quarter performance for fiscal year 2019. Following such presentation, a representative of Simpson Thacher reviewed with the Versum board of directors their fiduciary duties under applicable law.

On January 15, 2019, representatives of Lazard and Morgan Stanley spoke telephonically to discuss potential valuation methodologies with respect to a potential transaction between Entegris and Versum, in preparation for a meeting of Messrs. Novo, Ghasemi, Loy and Olson that had been scheduled for January 17, 2019.

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On January 16, 2019, the Entegris board of directors met telephonically, together with members of Entegris’ senior management and representatives of Morgan Stanley and Wachtell, Lipton, Rosen & Katz, counsel to Entegris, to receive an update on the status of the proposed transaction and to provide guidance to Messrs. Olson and Loy in advance of their meeting with Messrs. Novo and Ghasemi, Versum’s chief executive officer and chairman, respectively, scheduled for January 17, 2019. The directors authorized Mr. Loy to propose to Messrs. Novo and Ghasemi an exchange ratio of 1.097, meaning Versum stockholders would receive 1.097 shares of Entegris common stock for each share of Versum common stock held, which was the average of the exchange ratios implied by comparing the six calendar-month and one calendar-year volume weighted average trading prices of the two companies as of January 16, 2019, and authorized Mr. Loy to negotiate the exchange ratio further should the proposed 1.097 ratio not prove to be acceptable to Versum.

Based on the Entegris board of directors’ guidance, Entegris’ senior management determined to propose to Versum that the transaction be structured as an all-stock merger that would be intended to be tax-free to the stockholders of both companies, at an exchange ratio of 1.097 Entegris shares for each outstanding Versum share. On January 17, 2019, prior to the meeting of Messrs. Ghasemi, Novo, Loy and Olson, representatives of Morgan Stanley provided representatives of Lazard with a summary of Entegris’ proposed key terms, including, as had been previously discussed between the parties, that Mr. Ghasemi serve as chairman of the board of directors of the combined company, that Mr. Loy and Mr. Graves serve as the chief executive officer and chief financial officer of the combined company, respectively, and that the board of directors of the combined company include nine members, with four to be designated by Versum, four to be designated by Entegris, in addition to Mr. Loy as chief executive officer of the combined company.

Later that day, on January 17, 2019, Messrs. Novo, Ghasemi, Loy and Olson met in Scottsdale to discuss the potential transaction and certain key terms. The parties confirmed their continued belief that a merger of the two companies would be beneficial to both Entegris, Versum and their respective stockholders, and reached agreement, subject in both cases to board approval, as to certain terms regarding the governance of the combined company. After negotiation during the meeting, the participants agreed to recommend to their respective boards of directors an exchange ratio of 1.120 shares of Entegris common stock to be exchanged for each share of Versum common stock. The parties determined to proceed expeditiously to negotiate definitive transaction agreements and to complete due diligence, subject to further review and approval by the Entegris board of directors and the Versum board of directors.

On January 18, 2019, a meeting of the Versum board of directors, at which representatives of Versum’s senior management, Lazard, Simpson Thacher and Skadden, Arps, Slate, Meagher & Flom LLP, Versum’s antitrust counsel, which is referred to as Skadden (telephonically), were present, was held in Scottsdale, Arizona. At this meeting, Mr. Ghasemi provided the Versum board of directors with an update on the ongoing discussions and negotiations regarding the potential transaction with Entegris, including the proposed exchange ratio of 1.120 discussed by the parties on the previous day, and certain governance terms. Mr. Bitto then presented to the Versum board of directors an update on Versum’s financial performance in the first quarter of 2019 and discussion ensued. Following discussion, a representative of Simpson Thacher reviewed with the Versum board of directors their fiduciary duties under applicable law. Subsequently, representatives of Lazard reviewed with the Versum board of directors Lazard’s preliminary financial analysis of a potential transaction with Entegris. Members of the Versum board of directors then discussed with representatives of Versum senior management, Lazard and Simpson Thacher the due diligence process and the material terms of a merger agreement with Entegris, and representatives of Skadden reviewed with the Versum board of directors Skadden’s preliminary analysis of certain regulatory aspects of the proposed transaction.

On January 19, 2019, at the instruction of the Versum board of directors, Simpson Thacher provided an initial draft of a merger agreement to Wachtell Lipton.

From January 19 through January 27, 2019, Entegris, Versum and their respective financial and legal advisors continued to conduct due diligence on each other and the parties negotiated the terms of the merger

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agreement and ancillary documents and confirmed that Mr. Ghasemi would serve as the chairman of the board of the combined company, Mr. Loy would serve as the chief executive officer of the combined company, Mr. Graves would serve as the chief financial officer of the combined company and Michael Valente, the general counsel of Versum, would serve as the general counsel of the combined company.

On January 24, 2019, at a telephonic meeting of the Versum board of directors, at which representatives of Versum’s senior management, Lazard and Simpson Thacher were present, Messrs. Novo and Ghasemi updated the Versum board of directors on the status of the discussions with Entegris, the due diligence process, and the negotiations concerning material provisions of the merger agreement. A representative of Simpson Thacher then reviewed with the Versum board of directors their fiduciary duties under applicable law. Subsequently, representatives of Lazard reviewed with the Versum board of directors Lazard’s updated financial analysis of a potential transaction with Entegris based upon the proposed exchange ratio. Representatives of Simpson Thacher next discussed the revised merger agreement received from Entegris’ counsel, including certain remaining open issues. Mr. Novo then reviewed with the Versum board of directors the proposed communications plan with respect to the announcement of the potential transaction.

On January 25, 2019, the Entegris board of directors met in Boston, together with members of Entegris’ senior management and representatives of Morgan Stanley and Wachtell Lipton, to discuss and deliberate on the proposed combination of Entegris and Versum, and to receive presentations from Entegris’ senior management and advisors. Mr. Loy and representatives of Morgan Stanley and Wachtell Lipton briefed the directors on the status of negotiations with Versum, and members of Entegris senior management reviewed the results of Entegris’ business, synergy potential and due diligence review of Versum. Representatives of Morgan Stanley provided an updated preliminary financial analysis with respect to the potential transaction. A representative of Wachtell Lipton reviewed the directors’ fiduciary duties and presented a detailed summary of the terms of the draft merger agreement and proposed financing commitment related to the transaction. After discussion among the directors, including as to the matters described in the section entitled “—Recommendation of the Entegris Board of Directors; Entegris’ Reasons for the Merger” beginning on page 87, it was the unanimous view of the Entegris board of directors that Entegris should seek to finalize the merger agreement and related documents with Versum, subject to final review and approval by the Entegris board of directors.

Between January 25 and January 27, 2019, Entegris and Versum and their respective legal advisors continued to negotiate the merger agreement and related documents and to finalize due diligence. On January 26 and 27, 2019, Messrs. Loy, Graves, Novo and Ghasemi and certain representatives of the parties’ respective financial and communications advisors met in person at the offices of Wachtell Lipton to plan for the public announcement of the transaction, subject to approval of the transaction by the parties’ respective boards of directors.

In the afternoon of January 27, 2019, the Entegris board of directors met telephonically, together with members of Entegris’ senior management and representatives of Morgan Stanley and Wachtell Lipton. Mr. Loy and a representative of Wachtell Lipton provided an update on developments since the previous meeting of the Entegris board of directors. The representatives of Wachtell Lipton summarized the minimal changes to the draft merger agreement since the January 25 meeting of the Entegris board of directors, and representatives of Morgan Stanley provided directors with its financial analysis with respect to the potential transaction with Versum. Morgan Stanley rendered for the benefit of the Entegris board of directors its oral opinion, subsequently confirmed in writing, on January 27, 2019 that as of such date and based upon and subject to the various assumptions made, procedures followed, matters considered, and qualifications and limitations on the scope of the review undertaken by Morgan Stanley as set forth in the written opinion, the exchange ratio pursuant to the merger agreement was fair from a financial point of view to Entegris. See the section entitled “—Opinion of Entegris’ Financial Advisor” beginning on page 96 for more information.

After discussions, including as to the matters discussed below in the section entitled “—Recommendation of the Entegris Board of Directors; Entegris’ Reasons for the Merger” beginning on page 87 the Entegris

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board of directors, by unanimous vote of all of its members, (1) determined that the merger agreement and the transactions contemplated thereby, including but not limited to the merger, the share issuance and the adoption of the amended and restated certificate of incorporation on the terms set forth in the merger agreement were fair to, and in the best interests of, Entegris and the holders of shares of Entegris common stock, (2) approved and declared advisable the merger agreement and the transactions contemplated thereby, including the merger, on the terms and subject to the conditions set forth in the merger agreement, (3) directed that the share issuance and the merger agreement be submitted to the holders of shares of Entegris common stock for their approval and adoption, and the amended and restated certificate of incorporation be submitted to the holders of shares of Entegris common stock for their adoption and (4) resolved to recommend that the holders of shares of Entegris common stock vote in favor of the adoption of the merger agreement and the approval of the share issuance and in favor of the adoption of the amended and restated certificate of incorporation.

On January 27, 2019, a meeting of the Versum board of directors, at which representatives of Versum’s senior management, Lazard (telephonically) and Simpson Thacher (telephonically) were present, was held in Scottsdale, Arizona. Messrs. Ghasemi and Novo participated telephonically from Lazard’s offices in New York. Mr. Ghasemi updated the Versum board of directors on the status of the negotiations with Entegris, including that the parties had substantially finalized the merger agreement and resolved all significant open substantive issues with respect to the terms of the potential transaction. Mr. Ghasemi reported that the Entegris board of directors had met earlier on January 27, 2019 and had unanimously resolved to approve the potential transaction. A representative of Simpson Thacher then reviewed with the Versum board of directors their fiduciary duties under applicable law and the terms of the draft merger agreement. Representatives of Lazard then reviewed with the Versum board of directors Lazard’s financial analysis of the potential transaction with Entegris. Following further discussion, representatives of Lazard then delivered to the Versum board of directors Lazard’s oral opinion, subsequently confirmed in writing by delivery of Lazard’s opinion dated as of the same date, to the effect that, as of such date, and based upon and subject to the assumptions, procedures, factors, qualifications and limitations set forth in Lazard’s written opinion, the exchange ratio of 1.120 shares of Entegris common stock per share of Versum common stock was fair, from a financial point of view, to the Versum stockholders (other than the holders of Versum excluded shares), as further described in the section entitled “—Opinion of Versum’s Financial Advisor” beginning on page 107.

After discussions, including as to the matters discussed below in the section entitled “—Recommendation of the Versum Board of Directors; Versum’s Reasons for the Merger” beginning on page 91, the Versum board of directors unanimously determined that the merger agreement, the merger and the other transactions contemplated by the merger agreement were fair to, and in the best interests of, Versum and its stockholders, approved and declared advisable the merger agreement, the merger and the other transactions contemplated by the merger agreement, directed that the merger agreement be submitted for adoption at a meeting of Versum stockholders and resolved to recommend that the Versum stockholders vote in favor of the adoption of the merger agreement.

Following the approval of the merger and the merger agreement by the Entegris board of directors and the Versum board of directors, Entegris and Versum finalized and executed the merger agreement on January 27, 2019, and in the morning of January 28, 2019, prior to the opening of trading, issued a joint press release announcing the execution of the merger agreement.

On February 27, 2019, Dr. Stefan Oschmann, Chief Executive Officer of Merck, placed a call to Mr. Ghasemi to inform him of Merck’s interest in acquiring Versum. Mr. Ghasemi responded that he would discuss the call with the members of the Versum board of directors. Within minutes after such call, on February 27, 2019, Merck sent a letter to the Versum board of directors setting forth a non-binding unsolicited proposal to acquire Versum for $48.00 per share in cash, which is referred to as the Merck proposal.

Later that morning on February 27, 2019, Versum issued a press release that acknowledged receipt of the Merck proposal and stated that Versum continues to believe in the strategic and financial rationale of the proposed merger of equals with Entegris and that the Versum board of directors intends to thoroughly review the Merck proposal consistent with its fiduciary duties.

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On February 27, 2019, the Versum board of directors met telephonically, together with members of Versum’s senior management and representatives of Lazard and Simpson Thacher, to discuss next steps in connection with the review of the Merck proposal. Also on February 27, 2019, Entegris issued a press release in response to the Merck proposal, stating that Entegris believes the merger of Versum and Entegris will deliver meaningful value to Versum stockholders and Entegris stockholders.

On February 27, 2019, the Entegris board of directors held an update call, together with members of Entegris’ senior management and representatives of Morgan Stanley and Wachtell Lipton, to discuss developments as well as potential public disclosure of additional potential synergies identified during ongoing integration-planning discussions between Entegris and Versum.

On February 28, 2019, the Versum board of directors met via a telephonic meeting, at which representatives of Versum’s senior management, Lazard and Simpson Thacher were present. Following discussion of the Merck proposal and, in light of recent unusual trading activity in the stock of Versum, the Versum board of directors resolved to adopt a limited duration shareholder rights plan, which the Versum board of directors had previously reviewed with outside counsel and which was available to be implemented quickly in response to a specific threat to corporate policymaking, such as the decision to pursue the Entegris merger. Later on February 28, 2019, the Versum board of directors authorized and declared a dividend of one preferred share purchase right for each outstanding share of Versum common stock, payable to Versum stockholders of record on March 11, 2019, pursuant to the Versum rights plan (as defined in the section entitled “Comparison of Stockholders’ Rights—Stockholder Rights Plans” beginning on page 190).

On March 1, 2019, the Versum board of directors met telephonically, together with members of Versum’s senior management and representatives of Lazard and Simpson Thacher, to discuss the Merck proposal. Representatives of Lazard reviewed with the Versum board of directors Lazard’s financial analysis of the Merck proposal. Members of the Versum board of directors then discussed the terms of the Merck proposal and the merger agreement with Entegris, consulted with representatives of Versum senior management, Lazard and Simpson Thacher, and concluded, after careful review and consideration, that the Merck proposal was not a superior proposal for purposes of the merger agreement with Entegris. Later that day on March 1, 2019, Versum issued a press release announcing the determination by the Versum board of directors, and stating that Versum remains committed to completing the merger with Entegris, which the Versum board of directors believes will create significant long-term value, and is in the best interests of the Versum stockholders.

On March 2, 2019, the Versum board of directors met telephonically with members of Versum’s senior management and representatives of Simpson Thacher to discuss the desirability of obtaining advice from an additional financial advisor in connection the Merck proposal. Following discussion, the Versum board of directors instructed Versum’s senior management to seek to engage Citigroup Global Markets, which is referred to as Citi, to advise the Versum board of directors with respect to a financial analysis of the Merck proposal. Citi was selected on the basis of its experience in the industries in which Versum operates and its familiarity with Versum.

On March 4, 2019, representatives of Merck and its financial advisor proposed to arrange a telephone call between Dr. Oschmann of Merck and Mr. Ghasemi, which Mr. Ghasemi declined following consultation with members of Versum’s senior management and representatives of Simpson Thacher, consistent with the terms of the merger agreement.

On March 5, 2019, Merck issued an open letter to the Versum stockholders, stating that Merck remains fully committed to pursuing the Merck proposal and requesting that the Versum board of directors engage with Merck.

On March 6, 2019, the Versum board of directors met telephonically, together with members of Versum’s senior management and representatives of Citi and Simpson Thacher, to discuss the Merck proposal. At the meeting, representatives of Versum’s senior management updated the Versum board of directors on the status of

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the integration planning meetings between the management teams of Versum and Entegris and the identification of potential additional synergies resulting from the merger. Representatives of Citi presented to the Versum board of directors Citi’s preliminary financial analysis. Following discussion of the Merck proposal and the merger with representatives of Versum’s senior management, Citi and Simpson Thacher, the Versum board of directors reaffirmed, after careful review and consideration, its previous determination that the Merck proposal was not a superior proposal for purposes of the merger agreement with Entegris.

On March 8, 2019, Versum and Entegris issued a joint open letter to Versum stockholders and Entegris stockholders, reiterating the value-creation potential and strategic benefits of the merger and announcing additional expected synergies resulting from the merger. See the section entitled “—Certain Estimated Synergies” beginning on page 121 for a summary of such additional expected synergies.

Also on March 8, 2019, the Versum board of directors issued a statement reiterating its determination that the Merck proposal was not a superior proposal for purposes of the merger agreement with Entegris, and reaffirming its commitment to completing the merger with Entegris.

On March 11, 2019, Entegris and Versum announced the expiration of the waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended, in connection with the merger, which occurred at 11:59 p.m. EST on March 8, 2019 and satisfied one of the conditions to closing of the merger.

On March 12, 2019, Merck filed a preliminary proxy statement with the SEC with respect to the solicitation of proxies by Merck in opposition to the merger.

On March 14, 2019, Versum announced that it had entered into an amendment to the Versum rights plan to remove references to the defined concept of “Acting in Concert” from the Versum rights plan.

Recommendation of the Entegris Board of Directors; Entegris’ Reasons for the Merger

At a special meeting held on January 27, 2019, the Entegris board of directors unanimously: (1) determined that the merger agreement and the transactions contemplated thereby, including but not limited to the merger, the share issuance and the adoption of the amended and restated certificate of incorporation of Entegris on the terms set forth in the merger agreement, are fair to, and in the best interests of, Entegris and the holders of shares of Entegris common stock; (2) approved and declared advisable the merger agreement and the transactions contemplated thereby, including the merger, on the terms and subject to the conditions set forth in the merger agreement; (3) directed that the share issuance and the merger agreement be submitted to the holders of shares of Entegris common stock for their approval and adoption, and the amended and restated certificate of incorporation of Entegris be submitted to the holders of shares of Entegris common stock for their adoption; and (4) resolved to recommend that the holders of shares of Entegris common stock vote in favor of the adoption of the merger agreement, including the approval of the share issuance, and the adoption of the amended and restated certificate of incorporation of Entegris. Accordingly, the Entegris board of directors unanimously recommends that Entegris stockholders vote “FOR” the Entegris merger agreement proposal, “FOR” the Entegris charter proposal, “FOR” the Entegris compensation proposal and “FOR” the Entegris adjournment proposal.

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In reaching its determinations and recommendations, the Entegris board of directors consulted with Entegris’ senior management and its outside legal and financial advisors, and considered a number of factors, including the following factors that weighed in favor of the merger.

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The Entegris board of directors weighed these advantages and opportunities against a number of potentially negative factors in its deliberations concerning the merger agreement and the merger, including:

The Entegris board of directors considered all of these factors as a whole and, on balance, concluded that the potential benefits of the merger outweighed the risks and uncertainties of the merger.

In addition, the Entegris board of directors was aware of and considered the interests of its directors and executive officers that are different from, or in addition to, the interests of Entegris stockholders generally described in the section entitled “Interests of Entegris’ Directors and Executive Officers in the Merger” beginning on page 170.

The foregoing discussion of the information and factors that the Entegris board of directors considered is not intended to be exhaustive, but rather is meant to include the material factors that the Entegris board of directors considered. The Entegris board of directors collectively reached the conclusion to adopt the merger agreement and approve the amended and restated certificate of incorporation of Entegris, the merger and the other transactions contemplated by the merger agreement in light of the various factors described above and other factors that the members of the Entegris board of directors believed were appropriate. In view of the complexity and wide variety of factors, both positive and negative, that the Entegris board of directors considered in connection with its evaluation of the merger, the Entegris board of directors did not find it practical, and did not

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attempt, to quantify, rank or otherwise assign relative or specific weights or values to any of the factors it considered in reaching its decision and did not undertake to make any specific determination as to whether any particular factor, or any aspect of any particular factor, was favorable or unfavorable to the ultimate determination of the Entegris board of directors. In considering the factors discussed above, individual directors may have given different weights to different factors.

The foregoing description of Entegris’ consideration of the factors supporting the merger is forward-looking in nature. This information should be read in light of the factors discussed in the section entitled “Cautionary Statement Regarding Forward-Looking Statements” beginning on page 41.

Recommendation of the Versum Board of Directors; Versum’s Reasons for the Merger

At a meeting held on January 27, 2018, the Versum board of directors unanimously: