Proxy Statement Pursuant to Section 14(a) of the Securities
Exchange Act of 1934 (Amendment No.     )
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o  Preliminary Proxy Statement
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þ  Definitive Additional Materials
o  Soliciting Material Pursuant to §240.14a-11(c) or §240.14a-12
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MKS Instruments, Inc.
(Name of Registrant as Specified In Its Charter)
(Name of Person(s) Filing Proxy Statement)
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MKS Instruments, Inc. (the “Company”) has amended its Proxy Statement, dated March 23, 2009 as follows. The Company has revised one feature of its proposed amendment to the Company’s 2004 Stock Incentive Plan which allows for an option exchange program. The Company has determined that any and all options surrendered to the Company pursuant to the option exchange program shall not become available for the grant of future awards under the 2004 Stock Incentive Plan or any other Company plan.