Exhibit 8.1

WACHTELL, LIPTON, ROSEN & KATZ

MARTIN LIPTON HERBERT M. WACHTELL THEODORE N. MIRVIS EDWARD D. HERLIHY DANIEL A. NEFF ANDREW R. BROWNSTEIN MARC WOLINSKY STEVEN A. ROSENBLUM JOHN F. SAVARESE SCOTT K. CHARLES JODI J. SCHWARTZ ADAM O. EMMERICH RALPH M. LEVENE RICHARD G. MASON DAVID M. SILK ROBIN PANOVKA DAVID A. KATZ ILENE KNABLE GOTTS JEFFREY M. WINTNER TREVOR S. NORWITZ BEN M. GERMANA ANDREW J. NUSSBAUM	RACHELLE SILVERBERG STEVEN A. COHEN DEBORAH L. PAUL DAVID C. KARP RICHARD K. KIM JOSHUA R. CAMMAKER MARK GORDON JOSEPH D. LARSON JEANNEMARIE O’BRIEN WAYNE M. CARLIN STEPHEN R. DiPRIMA NICHOLAS G. DEMMO IGOR KIRMAN JONATHAN M. MOSES T. EIKO STANGE JOHN F. LYNCH WILLIAM SAVITT ERIC M. ROSOF GREGORY E. OSTLING DAVID B. ANDERS ANDREA K. WAHLQUIST	51 WEST 52ND STREET NEW YORK, N.Y. 10019-6150 TELEPHONE: (212) 403-1000 FACSIMILE: (212) 403-2000		ADAM J. SHAPIRO NELSON O. FITTS JOSHUA M. HOLMES DAVID E. SHAPIRO DAMIAN G. DIDDEN IAN BOCZKO MATTHEW M. GUEST DAVID E. KAHAN DAVID K. LAM BENJAMIN M. ROTH JOSHUA A. FELTMAN ELAINE P. GOLIN EMIL A. KLEINHAUS KARESSA L. CAIN RONALD C. CHEN GORDON S. MOODIE DONGJU SONG BRADLEY R. WILSON GRAHAM W. MELI GREGORY E. PESSIN CARRIE M. REILLY	MARK F. VEBLEN VICTOR GOLDFELD EDWARD J. LEE BRANDON C. PRICE KEVIN S. SCHWARTZ MICHAEL S. BENN SABASTIAN V. NILES ALISON ZIESKE PREISS TIJANA J. DVORNIC JENNA E. LEVINE RYAN A. McLEOD ANITHA REDDY JOHN L. ROBINSON JOHN R. SOBOLEWSKI STEVEN WINTER EMILY D. JOHNSON JACOB A. KLING RAAJ S. NARAYAN VIKTOR SAPEZHNIKOV MICHAEL J. SCHOBEL ELINA TETELBAUM
		GEORGE A. KATZ (1965-1989) JAMES H. FOGELSON (1967-1991) LEONARD M. ROSEN (1965-2014)
		OF COUNSEL
		WILLIAM T. ALLEN MARTIN J.E. ARMS MICHAEL H. BYOWITZ GEORGE T. CONWAY III KENNETH B. FORREST SELWYN B. GOLDBERG PETER C. HEIN MEYER G. KOPLOW LAWRENCE S. MAKOW DOUGLAS K. MAYER MARSHALL L. MILLER PHILIP MINDLIN ROBERT M. MORGENTHAU DAVID S. NEILL	HAROLD S. NOVIKOFF LAWRENCE B. PEDOWITZ ERIC S. ROBINSON PATRICIA A. ROBINSON* ERIC M. ROTH PAUL K. ROWE DAVID A. SCHWARTZ MICHAEL J. SEGAL ELLIOTT V. STEIN WARREN R. STERN PAUL VIZCARRONDO, JR. PATRICIA A. VLAHAKIS AMY R. WOLF
		* ADMITTED IN THE DISTRICT OF COLUMBIA
		COUNSEL
		DAVID M. ADLERSTEIN AMANDA K. ALLEXON LOUIS J. BARASH FRANCO CASTELLI DIANNA CHEN ANDREW J.H. CHEUNG PAMELA EHRENKRANZ KATHRYN GETTLES-ATWA ADAM M. GOGOLAK	NANCY B. GREENBAUM MARK A. KOENIG LAUREN M. KOFKE J. AUSTIN LYONS ALICIA C. McCARTHY PAULA N. RAMOS NEIL M. SNYDER S. CHRISTOPHER SZCZERBAN JEFFREY A. WATIKER

March 18, 2019

Entegris, Inc.

129 Concord Road

Billerica, Massachusetts 01821

Re: Versum Materials/Entegris Merger

Ladies and Gentlemen:

We have acted as counsel to Entegris Inc., a Delaware corporation (“Entegris”), in connection with the Merger, as defined and described in the Agreement and Plan of Merger, dated as of January 27, 2019 (the “Merger Agreement”), by and between Versum Materials, Inc., a Delaware corporation (“Versum”) and Entegris. For purposes of this opinion, capitalized terms used and not otherwise defined herein shall have the meaning ascribed thereto in the Merger Agreement. This opinion is being delivered in connection with the filing of the registration statement on Form S-4 (as amended, the “Registration Statement”) filed by Entegris, including the joint proxy statement/information statement and prospectus forming a part thereof, with the Securities and Exchange Commission under the Securities Act of 1933, as amended, relating to the proposed transactions pursuant to the Merger Agreement and to which this opinion appears as an exhibit.

We have examined (i) the Merger Agreement, (ii) the Registration Statement and (iii) the representation letters of Versum and Entegris delivered to us in connection with this opinion (the “Representation Letters”). In addition, we have examined, and relied as to matters of fact upon, originals or copies, certified or otherwise identified to our satisfaction, of such records, agreements, documents and other instruments and made such other inquiries as we have deemed necessary or appropriate to enable us to render the opinion set forth below. In such

WACHTELL, LIPTON, ROSEN & KATZ

Entegris Inc.

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examination, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies, and the authenticity of the originals of such latter documents. We have not, however, undertaken any independent investigation of any factual matter set forth in any of the foregoing.

In rendering such opinion, we have assumed, with your permission, that (i) the Merger will be effected in accordance with the Merger Agreement, (ii) the statements concerning the Merger set forth in the Merger Agreement and the Registration Statement are true, complete and correct and will remain true, complete and correct at all times up to and including the Effective Time, (iii) the representations made by Versum and Entegris in their respective Representation Letters are true, complete and correct and will remain true, complete and correct at all times up to and including the Effective Time and (iv) any representations made in the Merger Agreement or the Representation Letters “to the knowledge of”, or based on the belief of Versum and Entegris or similarly qualified are true, complete and correct and will remain true, complete and correct at all times up to and including the Effective Time, in each case without such qualification. We have also assumed that the parties have complied with and, if applicable, will continue to comply with, the covenants contained in the Merger Agreement.

Based upon the foregoing, and subject to the qualifications, assumptions and limitations stated herein and in the Registration Statement, we are of the opinion that the statements made in the Registration Statement under the caption “Material U.S. Federal Income Tax Consequences,” insofar as they purport to constitute summaries of certain provisions of U.S. federal income tax law and regulations or legal conclusions with respect thereto, constitute accurate summaries of such matters described therein in all material respects.

We express our opinion herein only as to those matters specifically set forth above and no opinion should be inferred as to the tax consequences of the Merger under any state, local or foreign law, or with respect to other areas of U.S. federal taxation. We do not express any opinion herein concerning any law other than the federal income tax law of the United States. Our opinion is based on current provisions of the Internal Revenue Code of 1986, as amended, Treasury Regulations promulgated thereunder, published pronouncements of the Internal Revenue Service and case law, any of which may be changed at any time with retroactive effect. Any change in applicable law or the facts and circumstances surrounding the Merger, or any inaccuracy in the statements, facts, or assumptions upon which we have relied, may affect the continuing validity of our opinion as set forth herein. We assume no responsibility to inform Entegris of any such change or inaccuracy that may occur or come to our attention.

We hereby consent to the filing of this opinion as Exhibit 8.1 to the Registration Statement, and to the references to our firm name therein. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act of 1933, as amended.

Very truly yours,

/s/ Wachtell, Lipton, Rosen & Katz