SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 11-K

(Mark One)

x Annual report pursuant to Section 15(d) of the Securities Exchange Act of 1934

For the fiscal year end: December 31, 2011

¨ Transition report pursuant to Section 15(d) of the Securities Exchange Act of 1934

For the transition period from            to

Commission file number: 000-08135

A. Full title of the plan and the address of the plan, if different from that of the issuer named below:

Sigma-Aldrich 401(k) Retirement Savings Plan

B. Name of issuer of the securities held pursuant to the plan and the address of its principal executive office:

Sigma-Aldrich Corporation

3050 Spruce Street

St. Louis, MO 63103

SIGMA-ALDRICH CORPORATION

401(k) RETIREMENT SAVINGS PLAN

Table of Contents and Definitions

	Page
Table of Contents:
Report of Independent Registered Public Accounting Firm		1
Statements of Net Assets Available for Benefits, December 31, 2011 and 2010		2
Statements of Changes in Net Assets Available for Benefits, For the Years Ended December  31, 2011 and 2010		3
Notes to Financial Statements, December 31, 2011 and 2010		4
Schedule:
1 Schedule H, Line 4i-Schedule of Assets (Held at End of Year), December 31, 2011		10

Definitions:

Plan	–	Sigma-Aldrich Corporation 401(k) Retirement Savings Plan
Trustee	–	Fidelity Management Trust Company
ERISA	–	Employee Retirement Income Security Act of 1974
Company	–	Sigma-Aldrich Corporation
Plan Administrator	–	Sigma-Aldrich Corporation
IRC	–	Internal Revenue Code
IRS	–	Internal Revenue Service

SIGMA-ALDRICH CORPORATION

401(k) RETIREMENT SAVINGS PLAN

Statements of Net Assets Available for Benefits

December 31, 2011 and 2010

	2011			2010
Assets:
Investments, at fair value	$	324,886,667		$	319,666,506
Notes Receivable from participants		6,980,461			6,851,945
Total assets		331,867,128			326,518,451
Liabilities:
Excess contributions payable to participants		---			(55,874	)
Total liabilities		---			(55,874	)
Net assets available for benefits, at fair value		331,867,128			326,462,577
Adjustment from fair value to contract value for interest in collective trust relating to fully benefit-responsive investment contract		(80,781	)		(11,708	)
Net assets available for benefits	$	331,786,347		$	326,450,869

The accompanying notes are an integral part of these financial statements.

2

SIGMA-ALDRICH CORPORATION

401(k) RETIREMENT SAVINGS PLAN

Statements of Changes in Net Assets Available for Benefits

Years ended December 31, 2011 and 2010

	2011			2010
Additions:
Additions to net assets attributed to:
Investment income (loss):
Net appreciation (depreciation) in fair value of investments	$	(15,140,104	)	$	34,864,189
Dividends		6,746,842			6,031,752
Interest		5,148			7,857
Total investment earnings (loss)		(8,388,114	)		40,903,798
Interest income on notes receivable from participants		290,300			329,052
Contributions:
Employee contributions		19,498,997			17,928,369
Employer contributions		9,188,482			9,260,010
Rollovers		3,524,602			675,212
Total contributions		32,212,081			27,863,591
Total additions		24,114,267			69,096,441
Deductions:
Deductions from net assets attributed to:
Benefit payments to participants	$	(18,731,887	)	$	(24,233,636	)
Administrative expenses		(46,902	)		(50,138	)
Total deductions		(18,778,789	)		(24,283,774	)
Increase in net assets available for benefits		5,335,478			44,812,667
Net assets available for benefits, beginning of year		326,450,869			281,638,202
Net assets available for benefits, end of year	$	331,786,347		$	326,450,869

The accompanying notes are an integral part of these financial statements.

3

SIGMA-ALDRICH CORPORATION

401(k) RETIREMENT SAVINGS PLAN

Notes to Financial Statements

December 31, 2011 and 2010

(1) Description of Plan

The following description of the Plan provides only general information. Participants should refer to the Plan document for a more complete description of the Plan's provisions.

(a) General

The Plan is a defined contribution retirement plan covering eligible employees of the Company and its domestic subsidiaries. The Plan includes the required provisions relating to eligibility, fiduciary standards, and other technical provisions under ERISA.

(b) Contributions

Employees not considered highly compensated may contribute 1% - 75% of eligible earnings, not to exceed the IRS dollar limitation. Employees that are highly compensated (defined as employees receiving compensation greater than $110,000) may contribute 1% - 15% of their eligible earnings, not to exceed the IRS dollar limitation. The Company contributes a match that equals 60% of the first 6% of the deferral percentage times the participant's compensation. Catch-up contributions are not eligible for matching. The Company also makes a fixed contribution of $150 per full-time participant and $75 per part-time participant each quarter.

New employees shall be deemed to have elected to make a salary deferral of 6% of compensation, generally effective 45 days after date of hire, unless the employee files an election to contribute a different amount than the 6% automatic deferral percentage before this date.

(c) Participant Accounts

Each participant's account is credited with the participant's deferral contribution and rollover contributions and allocations of (a) the Company's matching contributions, and (b) Plan earnings. Allocations are based on participant earnings or account balances, as defined. The participant's account is debited with any distributions or withdrawals made to the participant. The benefit to which a participant is entitled is the benefit that can be provided from the participant's vested account.

(d) Forfeitures

Forfeitures resulting from participant withdrawals before full vesting are used to reduce future employer contributions. Forfeited nonvested accounts at year end totaled $58,037 and $556,888 in 2011 and 2010, respectively. During the years ended December 31, 2011 and 2010 employer contributions were reduced by $736,728 and $-0-, respectively, from forfeited nonvested accounts.

(e) Vesting

Participants are always fully vested in participant contributions. The Company's matching contribution for participants becomes vested as follows:

Completed years of service	Vested percentage
Less than 3		0	%
3 or more		100	%

(f) Notes Receivable from Participants

Participants may borrow from their accounts a minimum of $500 up to a maximum equal to the lesser of $50,000 or 50% of their vested account balance. The loans are secured by the balance in the participant's account and bear interest at rates that range from 3.25% to 8.75%, which are commensurate with local prevailing rates as determined periodically by the investment committee. Loan terms range from one to five years, or longer for the purchase of a primary residence. Principal and interest payments are made by participants through payroll deductions. Notes receivable from participants are valued at their unpaid principal balance plus any accrued but unpaid interest.

(Continued)

4

SIGMA-ALDRICH CORPORATION

401(k) RETIREMENT SAVINGS PLAN

Notes to Financial Statements

December 31, 2011 and 2010

(g) Payment of Benefits

The Plan provides that a participant’s vested interest in the Plan will be distributed to the participant upon retirement, termination, total disability or death. Employees terminating employment with the Company for other reasons are entitled to their vested interest in the Plan. Participants with vested account balances in excess of $1,000 may elect to defer distribution to a future date as more fully described in the Plan. Participants may elect to (a) receive a single lump sum amount equal to their vested account balance, (b) have all of his or her distributions paid in the form of a direct trustee to trustee transfer to another eligible retirement plan designed by the participants, or (c) elect to receive his or her account balance in monthly, quarterly, or annual installments over any period not exceeding ten years.

Fully vested participants may also withdraw, if approved, certain basic contributions only in the event of a financial hardship as defined by the Plan and the IRC. A participant who receives a distribution for hardship shall be prohibited from making elective deferrals and employee contributions under this and all other plans of the employer for a six month period after receipt of the distribution.

(2) Summary of Significant Accounting Policies

(a) Basis of Accounting

The accompanying financial statements have been prepared on the accrual basis of accounting except for benefit payments.

Investment contracts held by a defined-contribution plan are required to be reported at fair value. However, contract value is the relevant measurement attribute for that portion of the net assets available for benefits of a defined-contribution plan attributable to fully benefit-responsive investment contracts because contract value is the amount participants would receive if they were to initiate permitted transactions under the terms of the Plan. The Statements of Net Assets Available for Benefits present the fair value of the investment contracts as well as the adjustment of the fully benefit-responsive investment contracts from fair value to contract value. The Statements of Changes in Net Assets Available for Benefits are prepared on a contract value basis.

(b) Accounting Pronouncements

In January 2010, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2010-06, “Improving Disclosures about Fair Value Measurements” (“ASU 2010-06”). ASU 2010-06 requires disclosures on the amount and reason for transfers in and out of Level 1 and 2 recurring fair value measurements. The standard clarifies existing disclosure requirements on levels of disaggregation and disclosures about inputs and valuation techniques. The standard also requires disclosure of activities, on a gross basis, including purchases, sales, issuances, and settlements, in the reconciliation of Level 3 fair value recurring measurements. The disclosures regarding Level 1 and 2 fair value measurements and clarification of existing disclosures became effective for the December 31, 2010 plan year and did not have a material impact on the Plan's financial statements. The disclosures of the Level 3 recurring fair value measurements became effective for the December 31, 2011 plan year and did not have a material impact on the Plan's financial statements.

 In May 2011, the FASB issued ASU No. 2011-04, “Amendments to Achieve Common Fair Value Measurement and Disclosure Requirements in U.S. GAAP and IFRS” (“ASU 2011-04”), which amends Accounting Standards Codification 820, “Fair Value Measurements and Disclosures”. ASU 2011-04 requires categorization by level for items that are required to be disclosed at fair value and information about transfers between Level 1 and Level 2.  In addition, ASU 2011-04 provides guidance on measuring the fair value of financial instruments managed within a portfolio and the application of premiums and discounts on fair value measurements. ASU 2011-04 requires additional disclosure for Level 3 measurements regarding the sensitivity of fair value to changes in unobservable inputs and any interrelationships between those inputs. ASU 2011-04 is to be applied prospectively and will be effective for the December 31, 2012 plan year.  The adoption will not have a material effect on the Statements of Net Assets Available for Benefits and Statements of Changes in Net Assets Available for Benefits. 

(c) Use of Estimates

The preparation of the accompanying financial statements, in conformity with accounting principles generally accepted in the United States of America, requires the Plan Administrator to make estimates and assumptions that affect the reported amounts of assets and liabilities at the date of the financial statements as well as the reported amounts of revenues and expenses and disclosure of contingent assets and liabilities during the reporting period. Actual results could differ from those estimates.

(Continued)

5

SIGMA-ALDRICH CORPORATION

401(k) RETIREMENT SAVINGS PLAN

Notes to Financial Statements

December 31, 2011 and 2010

(d) Valuation of Investments and Income Recognition

Investments are reported at fair value. Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. See note 5 for discussion of fair value measurements.

Purchases and sales of securities are recorded on a trade-date basis. Interest income is recorded on the accrual basis. Dividends are recorded on the ex-dividend date. Net appreciation (depreciation) includes the Plan’s gains and losses on investments bought and sold as well as held during the year.

(e) Payment of Benefits

Benefit payments to participants are recorded upon distribution.

(f) Administrative Expenses

Costs of administering the Plan are paid by the Company and are not reimbursed by the Plan.

(g) Subsequent Events

The Plan has evaluated subsequent events through June 28, 2012, the date the financial statements were issued.

(3) Reclassifications

Certain reclassifications of prior year amounts have been made to conform to the current year presentation.

(4) Investments

The following table presents the fair value of investments representing greater than 5% of the Plan’s net assets:

	December 31
	2011		2010
Fidelity Freedom Fund 2020 – Class K	$	21,021,203	$	20,194,517
Fidelity Money Market Trust Retirement Money Market Portfolio		22,375,776		22,370,597
Fidelity Balanced Fund – Class K		25,979,664		25,752,941
Fidelity Diversified International Fund – Class K		17,160,917		20,547,544
Sigma-Aldrich Company Stock Fund		36,123,841		35,978,441
Fidelity Contrafund – Class K		45,813,520		46,557,991
PIMCO Total Return Inst CL		25,908,089		24,205,773
Fidelity Freedom Fund 2030 – Class K		20,574,318		20,594,179

During 2011 and 2010, the Plan’s investments (including realized and unrealized gains and losses on investments bought and sold, as well as held during the year) (depreciated) appreciated in value by $(15,140,104) and $34,864,189, respectively, as follows:

	2011			2010
Mutual funds	$	(13,651,880	)	$	26,132,109
Sigma-Aldrich Company Stock Fund		(1,488,224	)		8,732,080
	$	(15,140,104	)	$	34,864,189

(Continued)

6

SIGMA-ALDRICH CORPORATION

401(k) RETIREMENT SAVINGS PLAN

Notes to Financial Statements

December 31, 2011 and 2010

(5) Fair Value Measurements

FASB ASC 820, “Fair Value Measurements and Disclosures” (“FASB ASC 820”), provides the framework for measuring fair value. This framework provides a fair value hierarchy that prioritizes the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements) and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy under FASB ASC 820 are as follows:

Level 1 Fair Value Measurements

Inputs to the valuation methodology are unadjusted quoted prices for identical assets or liabilities in active markets that the Plan has the ability to access.

Level 2 Fair Value Measurements

Inputs to the valuation methodology include:

• quoted prices for similar assets or liabilities in active markets;

• quoted prices for identical or similar assets or liabilities in inactive markets;

• inputs other than quoted prices that are observable for the asset or liability; and

• inputs that are derived principally from or corroborated by observable market data by correlation or other means.

If the asset or liability has a specified (contractual) term, the Level 2 input must be observable for substantially the full term of the asset or liability.

Level 3 Fair Value Measurements

Inputs to the valuation methodology are unobservable and significant to the fair value measurement.

The asset or liability’s fair value measurement level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Valuation techniques used need to maximize the use of observable inputs and minimize the use of unobservable inputs.

Following is a description of the valuation methodologies used for assets measured at fair value. There have been no changes in the methodologies used at December 31, 2011 and 2010.

Common stocks: Valued at the closing price reported on the active market on which the individual securities are traded.

Mutual funds: Valued at the net assets value ("NAV") of shares held by the Plan at year-end.

Common Collective Trust: Valued at NAV as a practical expedient of fair value. The NAV is based on the underlying investments, which are traded on an active market. The Plan does not have any collective investment funds with unfunded commitments or with any redemption restrictions. There is no redemption notice period for the individual participants within the Plan, however, there is up to a twelve- month redemption notice required for this investment at the Plan level. The participants nor the Plan have any funding commitments related to this investment.

The preceding methods used to arrive at a fair value calculation may not be indicative of the net realizable value or reflective of future fair values. Furthermore, while the Plan believes its valuation methods are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different fair value measurement at the reporting date.

(Continued)

7

SIGMA-ALDRICH CORPORATION

401(k) RETIREMENT SAVINGS PLAN

Notes to Financial Statements

December 31, 2011 and 2010

The following tables set forth by level, within the fair value hierarchy, the Plan’s assets at fair value as of December 31, 2011 and 2010:

December 31, 2011						Fair Value Measurements
			Balance			(Level 1)		(Level 2)		(Level 3)
Mutual Funds
	Mid Cap		$		24,082,997	$	24,082,997	$	-	$		-
	Large Cap				59,904,330		59,904,330		-			-
	International				20,611,786		20,611,786		-			-
	Blended				132,604,554		132,604,554
	Bond				25,908,089		25,908,089		-			-
	Money Market				22,375,776		22,375,776		-			-
Total Mutual Funds					285,487,532		285,487,532		-			-
Common Stock Fund					36,123,841		36,123,841		-			-
Common Collective Trust					3,275,294		-		3,275,294			-
Total			$		324,886,667	$	321,611,373	$	3,275,294	$		-
December 31, 2010						Fair Value Measurements
			Balance			(Level 1)		(Level 2)		(Level 3)
Mutual Funds
		Mid Cap	$	24,871,348		$	24,871,348	$	-	$	-
		Large Cap		58,757,201			58,757,201		-		-
		International		24,212,737			24,212,737		-		-
		Blended		127,830,512			127,830,512
		Bond		24,205,773			24,205,773		-		-
		Money Market		22,370,597			22,370,597		-		-
Total Mutual Funds				282,248,168			282,248,168		-		-
Common Stock Fund				35,978,441			35,978,441		-		-
Common Collective Trust				1,439,897			-		1,439,897		-
Total			$	319,666,506		$	318,226,609	$	1,439,897	$	-

(Continued)

8

SIGMA-ALDRICH CORPORATION

401(k) RETIREMENT SAVINGS PLAN

Notes to Financial Statements

December 31, 2011 and 2010

(6) Investment in Common Collective Trust

The Fidelity Managed Income Portfolio (the “Managed Income Fund”), a common collective trust fund, invests in a variety of investments such as fixed income securities or bond funds and enters into wrapper contracts issued by a third party, such as a bank or insurance company, to make payments to a portfolio in certain circumstances. The WRAP issuer agrees to pay an amount sufficient to cover unit holder redemptions.

The interest-crediting rate is the periodic interest rate accrued to participants and is reset on a monthly basis to reflect the performance of the underlying securities.

Participants may ordinarily direct the withdrawal or transfer of all or a portion of their investment in the Managed Income Fund at contract value. Certain events may limit the ability of the Plan to transact at contract value with the issuer. The Plan Administrator does not believe that the occurrence of any such event is probable. For the Plan years ended December 31, 2011 and 2010, the average yield and crediting interest rates were approximately 1.92% and 2.68%, respectively.

(7) Tax Status

The IRS issued a favorable determination letter dated January 5, 2009 stating that the Plan is tax exempt under Section 401 of the IRC. Although the Plan has been amended since receiving the determination letter, the Plan Administrator and the Plan’s tax counsel believe that the Plan is designed and is currently being operated in compliance with the applicable requirements of the IRC.

Accounting principles generally accepted in the United States of America require Plan management to evaluate tax positions taken by the Plan and recognize a tax liability (or asset) if the Plan has taken an uncertain position that more likely than not would not be sustained upon examination by the IRS. The Plan Administrator has analyzed the tax positions taken by the Plan, and has concluded that as of December 31, 2011, there are no uncertain positions taken or expected to be taken that would require recognition of a liability (or asset) or disclosure in the financial statements. The Plan is subject to routine audits by taxing jurisdictions; however, there are currently no audits for any tax periods in progress. The Plan Administrator believes it is no longer subject to income tax examinations for years prior to 2008.

(8) Related Party

The Plan assets are maintained in a trust fund. The investments, and changes therein, of this trust fund have been reported to the Plan by the Trustee. Therefore, these transactions qualify as party-in-interest transactions. Fees paid by the Plan for the investment management services amounted to $46,902 and $50,138 for the years ended December 31, 2011 and 2010 respectively.

As of December 31, 2011 and 2010, investments include 547,798 shares and 513,089 shares of Sigma-Aldrich Corporation common stock, respectively, having an aggregate fair value of $34,215,463 and $34,151,204, respectively.

(9) Risks and Uncertainties

The Plan invests in various investment securities. Investment securities are exposed to various risks such as interest rate, market, and credit risks. Due to the level of risk associated with certain investment securities, it is at least reasonably possible that changes in the values of investment securities will occur in the near term and that such changes could materially affect participants’ account balances and the amounts reported in the statement of net assets available for benefits.

(10) Plan Termination

The Company intends to continue the Plan; however, the Company reserves the right to terminate the Plan. In the event the Plan terminates, participants will generally become fully vested in the employer match portion of their account balances.

(Continued)

9

Schedule I

SIGMA-ALDRICH CORPORATION

401(k) RETIREMENT SAVINGS PLAN

Schedule H, Line 4i—Schedule of Assets (Held at End of Year)

December 31, 2011

a)	b) Identity of issue, borrower, lessor or similar party	c) Description	d)	Cost	e)	Curent Value
	Mutual funds:
*	Fidelity Management Trust Company	Retirement Money Market Portfolio	$	22,375,776	$	22,375,776
*	Fidelity Management Trust Company	Balanced Fund – Class K		19,132,129		25,979,664
	PIMCO Total Return Inst CL	PIMCO Total Return Inst CL		25,926,204		25,908,089
*	Fidelity Management Trust Company	Low-Priced Stock Fund – Class K		7,250,136		9,067,763
*	Fidelity Management Trust Company	Diversified International Fund – Class K		14,725,510		17,160,917
*	Fidelity Management Trust Company	Freedom Income Fund – Class K		1,452,937		1,418,377
*	Fidelity Management Trust Company	Freedom 2000 Fund – Class K		1,121,224		1,092,769
*	Fidelity Management Trust Company	Freedom 2005 Fund – Class K		355,644		338,030
*	Fidelity Management Trust Company	Freedom 2010 Fund – Class K		4,547,526		4,280,855
*	Fidelity Management Trust Company	Freedom 2015 Fund – Class K		9,773,305		9,207,921
*	Fidelity Management Trust Company	Freedom 2020 Fund – Class K		22,550,341		21,021,203
*	Fidelity Management Trust Company	Freedom 2025 Fund – Class K		17,174,225		15,848,699
*	Fidelity Management Trust Company	Freedom 2030 Fund – Class K		22,444,507		20,574,318
*	Fidelity Management Trust Company	Freedom 2035 Fund – Class K		14,314,094		13,007,313
*	Fidelity Management Trust Company	Freedom 2040 Fund – Class K		11,090,828		10,068,558
*	Fidelity Management Trust Company	Freedom 2045 Fund – Class K		5,687,687		5,180,778
*	Fidelity Management Trust Company	Freedom 2050 Fund – Class K		5,036,396		4,586,069
*	Fidelity Management Trust Company	Small Cap Stock Fund		8,491,779		8,277,247
*	Fidelity Management Trust Company	Contrafund – Class K		31,662,209		45,813,520
	American Fund	American Funds Fundamental Investors Fund Class R5		3,656,251		3,736,810
	American Beacon Fund	American Beacon Lg Cap Value Inst CL		3,151,181		2,981,298
	Columbia Acorn	Columbia Acorn International Fund Class Z		3,724,583		3,450,869
	Goldman Sachs	Goldman Sachs Mid Cap Value Fund Institutional Class		4,519,207		4,540,171
	Munder	Munder Mid-Cap Core Growth Class Y		2,049,428		2,197,816
	Spartan	Spartan 500 Index Fund – Institutional Class		6,661,429		7,372,702
				268,874,536		285,487,532
	Common Stock:
*	Sigma-Aldrich	Sigma-Aldrich Company Stock Fund		24,653,971		36,123,841
	Common Collective Trust:
*	Fidelity Management Trust Company	Managed Income Portfolio		3,194,513		3,275,294
*	Notes Receivable from Participants	Range of interest rates 3.25% – 8.75%				6,980,461
					$	331,867,128

* Represents a party-in-interest.

See accompanying independent auditors’ report.

10

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, Sigma-Aldrich Corporation, as Plan Administrator of the Sigma-Aldrich 401(k) Retirement Savings Plan, has duly caused this annual report to be signed on its behalf by the undersigned hereunto duly authorized.

SIGMA-ALDRICH CORPORATION
401(k) RETIREMENT SAVINGS PLAN
By:	/s/ Mike Hollenkamp
	Mike Hollenkamp, Director of Global Treasury
	Sigma-Aldrich Corporation
June 28, 2012

11

EXHIBIT INDEX

Exhibit
23	Consent of Independent Registered Public Accounting Firm – Brown Smith Wallace LLC

12