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                                                                       Exhibit 5



                                HALE AND DORR LLP
                               Counsellors at Law

                  60 State Street, Boston, Massachusetts 02109
                         617-526-6000 * FAX 617-526-5000




                               February 28, 2000



MKS Instruments, Inc.
Six Shattuck Road
Andover, MA  01810

         Re:    MKS Instruments, Inc. International Employee Stock Purchase Plan
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Ladies and Gentlemen:

We have assisted in the preparation of a Registration Statement on Form S-8 (the
"Registration Statement") to be filed with the Securities and Exchange
Commission relating to 50,000 shares of common stock, no par value per share
(the "Shares"), of MKS Instruments, Inc., a Massachusetts corporation (the
"Company"), issuable under the Company's International Employee Stock Purchase
Plan (the "Plan").

We have examined the Restated Articles of Organization of the Company and the
Amended and Restated By-Laws of the Company, and originals, or copies certified
to our satisfaction, of all pertinent records of the meetings of the directors
and stockholders of the Company, the Registration Statement and such other
documents relating to the Company as we have deemed material for the purposes of
this opinion.

In examination of the foregoing documents, we have assumed the genuineness of
all signatures and the authenticity of all documents submitted to us as
originals, the conformity to original documents of all documents submitted to us
as certified, photostatic or facsimile copies, the authenticity of the originals
of such latter documents and the legal competence of all signatories to such
documents.

We assume that the appropriate action will be taken prior to the offer and sale
of the shares in accordance with the Plan to register and qualify the Shares for
sale under all applicable state securities or "blue sky" laws.

We express no opinion herein as to the laws of any state or jurisdiction other
than the laws of the Commonwealth of Massachusetts and the federal laws of the
United States of America.

Based upon and subject to the foregoing, we are of the opinion that the Company
has duly authorized for issuance the Shares covered by the Registration
Statement to be issued under the Plan, as described in the Registration
Statement, and such Shares, when issued in accordance with the terms of the
Plan, will be legally issued, fully paid and nonassessable.



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It is understood that this opinion is to be used only in connection with the
offer and sale of the Shares while the Registration Statement is in effect.

Please note that we are opining only as to the matters expressly set forth
herein, and no opinion should be inferred as to any other matters.

We hereby consent to the filing of this opinion with the Commission as an
exhibit to the Registration Statement in accordance with the requirements of
Item 601(b)(5) of Regulation S-K under the Securities Act of 1933, as amended
(the "Securities Act") and to the use of our name therein under the caption
"Interests of Named Experts and Counsel." In giving such consent, we do not
hereby admit that we are in the category of persons whose consent is required
under Section 7 of the Securities Act or the rules and regulations of the
Commission.

                                             Very truly yours,

                                             /s/ Hale and Dorr LLP

                                             HALE AND DORR LLP




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