Item 1.01 Entry into a Material Definitive Agreement.

On May 4, 2009, the shareholders of MKS Instruments, Inc. (the "Company") approved the following:

1) an amendment to the Company's 2004 Stock Incentive Plan to allow for a one-time option exchange program;

2) an amendment to the Company's Third Restated 1999 Employee Stock Purchase Plan increasing the number of shares available thereunder from 1,250,000 to 1,950,000 shares; and

3) an amendment to the Company's Second Amended and Restated International Employee Stock Purchase Plan increasing the number of shares available thereunder from 250,000 to 400,000 shares.

A copy of each of these plans, as amended, is filed as an exhibit to this report.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

10.1 2004 Stock Incentive Plan (as amended through May 4, 2009)
10.2 Fourth Amended Restated 1999 Employee Stock Purchase Plan (as amended through May 4, 2009)
10.3 Third Amended and Restated International Employee Stock Purchase Plan (as amended through May 4, 2009)

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