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 email was sent from Rakesh Sachdev to Sigma-Aldrich Corporation employees on October 7, 2014:

Greetings Colleagues,

I am writing today to update you on the announcement made two weeks ago in relationship to the Merck KGaA acquisition and to give you some insight into what’s to come in the weeks ahead. I can assure you that keeping you apprised of the status of the integration and anything you need to know to help you with the transition is a top priority for me personally and for my team.

I have recently visited several sites in the U.S. and have hosted a number of Town Hall Meetings. I have found these meetings to be very constructive. I know there are some questions we haven’t been able to answer to date, but I expect the answers will come with time. I am working closely with Corporate Communications and Human Resources to respond to your questions. I also plan to host similar Town Hall Meetings in Europe and in Asia-Pacific and I look forward to keeping the lines of communication open throughout this process.

That being said, until the deal closes, it is business as usual for all of us. We need to focus on our customers and on living our corporate values in the work we do every day. And during this time, which I know comes with some uncertainty, it’s imperative that we remember the fundamentals of our business that have led to our ongoing success including safety, quality and delivery.

The safety of our employees continues to be of the utmost importance. At times like this, it can be easy to get distracted, and that’s when accidents happen. Remember to be cautious and to stay focused on the task at hand. Our supply chain, like our people, is one of our greatest assets. In the coming weeks and months, we must continue to deliver our products on time and to meet our customers’ expectations for service. We must also adhere to the quality standards for which we are known. We want this transition to be seamless not only for our employees, but also for our customers.

I plan to send more From the Desk of Rakesh messages over the course of the coming months, and I hope to share more details on the integration in my next communication. In the meantime, I encourage you to keep an open dialogue with your managers and supervisors; they are here to help. Also, continue referring to the resources created particularly for the Merck KGaA acquisition, including the post announcement communications section of News of Note. We want you to be informed as we move toward the future. With each passing day, I am more excited about the potential of this combination and the opportunities it presents for our employees.

Best Regards,

Rakesh Sachdev

President and CEO

To ensure delivery, add mailuser@go.sigmaaldrich.com to your address book. Follow this link for instructions. Copyright ^© 2014 Sigma-Aldrich Co. LLC. All Rights Reserved. Reproduction of any materials in this e-mail is strictly forbidden without permission. Privacy Policy

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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 KGaA (“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.