Exhibit 5.1

             [Wilmer Cutler Pickering Hale and Dorr LLP Letterhead]

August 4, 2005

MKS Instruments, Inc.
90 Industrial Way
Wilmington, MA 01887

Re: 2004 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 (the "Commission") under the Securities Act of 1933, as amended (the
"Securities Act"), relating to an aggregate of 15,000,000 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 2004
Stock Incentive Plan (the "Plan").

We have examined the Articles of Organization and By-Laws of the Company, each
as amended and restated to date, 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 our examination of the foregoing documents, we have assumed the genuineness
of all signatures, 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 other copies, the authenticity of the originals of
any such 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 state laws of The Commonwealth of Massachusetts and the federal laws of
the United States of America.

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.



MKS Instruments, Inc.
August 4, 2005
Page 2

Based on the foregoing, we are of the opinion that the Shares have been duly
authorized for issuance and, when the Shares are issued and paid for in
accordance with the terms and conditions of the Plan, the Shares will be validly
issued, fully paid and nonassessable.

We hereby consent to the filing of this opinion with the Securities and Exchange
Commission in connection with the Registration Statement in accordance with the
requirements of Item 601(b)(5) of Regulation S-K under the Securities Act. 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,

WILMER CUTLER PICKERING
HALE AND DORR LLP

By: /s/ Mark G. Borden
   ----------------------------
   Mark G. Borden, a Partner