UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

(Rule 14a-101)

INFORMATION REQUIRED

IN PROXY STATEMENT

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No.     )

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¨ Preliminary Proxy Statement

¨ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

¨ Definitive Proxy Statement

¨ Definitive Additional Materials

x Soliciting Material under §240.14a-12

Sigma-Aldrich Corporation

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

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The following letter was sent from Rakesh Sachdev, President and Chief Executive Officer of Sigma-Aldrich Corporation (“Sigma-Aldrich”) to select Sigma-Aldrich dealers and distributors on September 24, 2014:

3050 Spruce Street, St. Louis, MO 63103 USA

Tel: (800) 521-8956 (314) 771-5765 Fax: (800) 325-5052 (314) 771-5757

September 24, 2014

TO: Sigma-Aldrich Dealers and Distributors

FROM: Rakesh Sachdev

SUBJECT: Important News from Sigma-Aldrich

Dear Valued Dealers and Distributors:

As an important Sigma-Aldrich partner, I am excited to share some significant news with you about our company. Sigma-Aldrich has entered into an agreement to be acquired by Merck KGaA, Darmstadt, Germany (“Merck”), a global pharmaceutical, chemical and life science company. The recent press release announcing the agreement is attached for your reference.

This combination brings together two leading life science businesses with complementary products and capabilities to establish an even stronger partner for you and your customers. Joining forces with Merck will enable the combined company to offer more choices to our customers in research, manufacturing and diagnostic and testing labs worldwide, including a broader range of products and capabilities, and enhanced global reach. Merck is a distinguished and values-driven industry leader that shares our commitment to product innovation, operational excellence and customer service. As part of a larger company with more than $6.1 billion in annual life science revenue (based on 2013 financials), the combined company will have an increased ability to develop and deliver breakthrough innovations and comprehensive solutions.

Notably, this announcement does not change our current relationship. You should expect the same levels of service and delivery from us moving forward, as well as a continued commitment to our partnership with you. Today’s news is just the first step in a multi-month process to complete this transaction, which we expect will be by mid-year 2015, subject to shareholder and regulatory approval and other customary closing conditions. Until that time, Sigma-Aldrich will continue to operate as an independent company, and our approach will be business-as-usual for our dealers, distributors, customers and other stakeholders.

Our relationship with partners like you and the customers we jointly serve is of the utmost of importance to us, and we will keep you informed as we work towards the completion of this transaction. In the meantime, please do not hesitate to reach out to your Sigma-Aldrich contacts with any questions or concerns you may have.

Best,

Rakesh Sachdev

Sigma-Aldrich President & CEO

# # #

This communication may include “forward-looking statements.” Statements that include words such as “anticipate,” “expect,” “should,” “would,” “intend,” “plan,” “project,” “seek,” “believe,” “will,” and other words of similar meaning in connection with future events or future operating or financial performance are often used to identify forward-looking statements. All statements in this communication, other than those relating to historical information or current conditions, are forward-looking statements. We intend these forward-looking statements to be covered by the safe harbor provisions for forward-looking statements in the Private Securities Litigation Reform Act of 1995. These forward-looking statements are subject to a number of risks and uncertainties, many of which are beyond the control of Sigma-Aldrich Corporation (“Sigma-Aldrich”) and Merck, which could cause actual results to differ materially from such statements.

Risks and uncertainties relating to the proposed transaction include, but are not limited to: the risk our shareholders do not approve the transaction; uncertainties as to the timing of the transaction; the risk that regulatory or other approvals required for the transaction are not obtained or are obtained subject to conditions that are not anticipated; competitive responses to the transaction; litigation relating to the transaction; uncertainty of the expected financial performance of the combined company following completion of the proposed transaction; Merck’s ability to achieve the cost-savings and synergies contemplated by the proposed transaction within the expected time frame; Merck’s ability to promptly and effectively integrate the businesses of Sigma-Aldrich and Merck; a potential downgrade in the rating of Merck’s or Sigma-Aldrich’s indebtedness; downward pressure on Merck’s or Sigma-Aldrich’s common stock price and its impact on goodwill impairment evaluations; the effects of the business combination of Merck and Sigma-Aldrich, including the combined company’s future financial condition, operating results, strategy and plans; and disruption from the proposed transaction making it more difficult to maintain relationships with customers, employees or suppliers.

The foregoing review of important factors should not be construed as exhaustive and should be read in conjunction with the other cautionary statements that are included elsewhere, including the Risk Factors section of Sigma-Aldrich’s most recent annual and quarterly reports on Form 10-K and Form 10-Q. Any forward-looking statements made in this communication are qualified in their entirety by these cautionary statements, and there can be no assurance that the actual results or developments anticipated by us will be realized or, even if substantially realized, that they will have the expected consequences to, or effects on, us or our business or operations. Except to the extent required by applicable law, we undertake no obligation to update publicly or revise any forward-looking statement, whether as a result of new information, future developments or otherwise.

Important Additional Information

This communication is being made in respect of the proposed merger transaction involving Sigma-Aldrich and Merck. The proposed merger will be submitted to the shareholders of Sigma-Aldrich for their consideration. In connection therewith, Sigma-Aldrich intends to file relevant materials with the Securities and Exchange Commission (the “SEC”), including a preliminary proxy statement and a definitive proxy statement. The definitive proxy statement will be mailed to the shareholders of Sigma-Aldrich. BEFORE MAKING ANY VOTING OR ANY INVESTMENT DECISION, INVESTORS AND SHAREHOLDERS ARE URGED TO READ THE DEFINITIVE PROXY STATEMENT REGARDING THE PROPOSED TRANSACTION AND ANY OTHER RELEVANT DOCUMENTS FILED OR TO BE FILED WITH THE SEC CAREFULLY AND IN THEIR ENTIRETY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED TRANSACTION. Investors and shareholders may obtain free copies of the proxy statement, any amendments or supplements thereto and other documents containing important information about Sigma-Aldrich, once such documents are filed with the SEC, through the website maintained by the SEC at www.sec.gov. Copies of the documents filed with the SEC by the Company will be available free of charge on Sigma-Aldrich’s website at http://investor.sigmaaldrich.com under the heading “Financial Information—SEC Filings”. Shareholders of Sigma-Aldrich may also obtain a free copy of the definitive proxy statement by contacting Sigma-Aldrich’s Investor Relations Department at (314) 898-4643.

Sigma-Aldrich and certain of its directors, executive officers and other members of management and employees may be deemed to be participants in the solicitation of proxies in connection with the proposed transaction. Information about the directors and executive officers of Sigma-Aldrich is set forth in its proxy statement for its 2014 annual meeting of shareholders, which was filed with the SEC on March 21, 2014, its annual report on Form 10-K for the fiscal year ended December 31, 2013, which was filed with the SEC on February 6, 2014, and in subsequent documents filed with the SEC, each of which can be obtained free of charge from the sources indicated above. Other information regarding the participants in the proxy solicitation of the shareholders of Sigma-Aldrich and a description of their direct and indirect interests, by share holdings or otherwise, will be contained in the preliminary and definitive proxy statements and other relevant materials to be filed with the SEC when they become available.