Item 1.01 Entry into a Material Definitive Agreement.

On March 18, 2009, MKS Instruments, Inc., MKS Japan, Inc. and HSBC Bank USA, National Association entered into a fifth amendment to the Optional Advance Demand Grid Note dated August 3, 2004 among such parties, as amended, (a) decreasing the maximum amount of the note from $35 million to $5 million, (b) decreasing the limit for standby letters of credit under the note from $750,000 to $650,000, and (c) establishing an annual facility fee of 0.0375% of the maximum amount of the note. MKS believes the reduced amount of the note more accurately reflects MKS' anticipated utilization of this line, and minimizes the cost of the new facility fee. As of December 31, 2008, MKS had outstanding borrowings of $1.1 million under this line. In addition, as of December 31, 2008, MKS and its subsidiaries had an aggregate of approximately $27.5 million available under other credit lines and short term borrowing arrangements, of which $16.7 million was outstanding on such date. As of December 31, 2008, MKS also had cash, cash equivalents and short-term investments, net of debt, of $259.8 million.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

99.1 Fifth Amendment, dated March 18, 2009, to Optional Advance Demand Grid Note dated August 3, 2004 (the "Note"), along with the Fourth Amendment to the Note, dated July 31, 2008 and the Third Amendment to the Note, dated July 31, 2007.

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