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SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d) OF

THE SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported): October 25, 2010

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SIGMA-ALDRICH CORPORATION

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(Exact Name of Registrant as Specified in its Charter)

3050 Spruce Street

St. Louis, Missouri 63103

(Address of Principal Executive Offices)

Registrant’s telephone number, including area code: (314) 771-5765

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Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

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Item 1.01. Entry into a Material Definitive Agreement.

On October 25, 2010, Sigma-Aldrich Corporation (the “Company”) entered into an Underwriting Agreement (the “Underwriting Agreement”) with Banc of America Securities LLC and J.P. Morgan Securities LLC, as Representatives of the several underwriters named therein (the “Underwriters”), pursuant to which the Company agreed to issue and sell to the Underwriters $300,000,000 aggregate principal amount of its 3.375% Senior Notes due November 1, 2020 (the “Notes”). The public offering price of the Notes was 99.915% of the principal amount. The Company expects net proceeds (before expenses) of approximately $297.8 million and to use the proceeds for general corporate purposes.

This offering is being made pursuant to the Company’s automatic shelf registration statement on Form S-3 (Registration

No. 333-170109) and a related prospectus supplement, each filed with the Securities and Exchange Commission.

The Underwriting Agreement includes customary representations, warranties and covenants by the Company. Under the terms of the Underwriting Agreement, the Company has agreed to indemnify the Underwriters against certain liabilities. The foregoing description of the Underwriting Agreement does not purport to be complete and is qualified in its entirety by reference to the full text of such document, a copy of which is attached to this Current Report on Form 8-K as Exhibit 1.1.

From time to time in the ordinary course of their respective businesses, certain of the Underwriters and their affiliates have engaged in and may in the future engage in commercial banking, derivatives and/or financial advisory, investment banking and other commercial transactions and services with the Company and its affiliates for which they have received or will receive customary fees and commissions.

Item 7.01. Regulation FD Disclosure

On October 25, 2010, the Company issued separate press releases announcing commencement and the pricing of the offering, copies of which are attached hereto as Exhibit 99.1 and 99.2, respectively.

Item 9.01. Financial Statements and Exhibits.

(d) Exhibits.

See Exhibit Index.

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SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Dated: October 26, 2010

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EXHIBIT INDEX

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