1

                                                                       EXHIBIT 5



                                HALE AND DORR LLP
                               Counsellors at Law

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



                                   May 7, 1999


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

         Re:      AMENDED AND RESTATED 1995 STOCK INCENTIVE PLAN

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 3,750,000 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 Amended and Restated 1995 Stock
Incentive 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.



   2
MKS Instruments, Inc.
May 7, 1999
Page 2


      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.

      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