EXHIBIT 23.1

            CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

      We hereby consent to the incorporation by reference in the Registration
Statements on Form S-8 (No. 333-78069, 333-78071, 333-78073, 333-31224,
333-54486, 333-54488, 333-54490, 333-90498, 333-90500, 333-90502, 333-116385,
333-116387 and 333-116389) and Form S-3 (No. 333-34450 and 333-109753) of MKS
Instruments, Inc. of our report dated March 14, 2005 relating to the
consolidated financial statements, the financial statement schedule,
management's assessment of the effectiveness of internal control over financial
reporting and the effectiveness of internal control over financial reporting,
which appears in this Form 10-K.

/s/ PricewaterhouseCoopers LLP

Boston, Massachusetts
March 14, 2005