Sec Form 4 Filing - Mahoney Robert Charles @ ON SEMICONDUCTOR CORP - 2012-05-03

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Mahoney Robert Charles 2. Issuer Name and Ticker or Trading Symbol ON SEMICONDUCTOR CORP [ ONNN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) Exec VP Sales & Marketing (Last) (First) (Middle) 5005 EAST MCDOWELL ROAD 3. Date of Earliest Transaction (MM/DD/YY) 05/03/2012 (Street) PHOENIX, AZ85008 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common 05/03/2012 A 58,478 ( 1 ) A $ 8.39 252,493 D Common 05/03/2012 F 18,768 ( 2 ) D $ 8.39 233,725 D Common 05/04/2012 S 60,000 D $ 8.4027 173,725 D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Mahoney Robert Charles 5005 EAST MCDOWELL ROAD PHOENIX, AZ85008 Exec VP Sales & Marketing Signatures Judith A. Boyle, Attorney-in-Fact 05/07/2012 Signature of Reporting Person Date

Explanation of Responses:

( 1 )This transaction reports the acquisition, effective May 3, 2012, of 58,478 shares of common stock resulting from the attainment of a performance goal under an earlier Performance Based Restricted Stock Units Award ("PBRSU Award"), as described more below. The Compensation Committee of the Issuer originally awarded 701,740 PBRSUs to the Reporting Person with an effective grant date of March 2, 2009, subject to the Issuers 2000 Stock Incentive Plan and a related award agreement. The PBRSU Award, which vested generally over a three-year (twelve quarter) period upon the Issuer achieving certain adjusted non-GAAP earnings before interest, taxes, depreciation and amortization ("EBITDA") base and target performance measurements as set forth in the PBRSU Award agreement, is now fully vested. ( 2 )This disposition transaction represents shares withheld by the Issuer pursuant to the Reporting Person's March 2, 2009 PBRSU award. On May 3, 2012, certain of these units vested. These shares were withheld to satisfy the Reporting Person's withholding obligations for various taxes and similar items. The Issuer will pay these taxes on behalf of the Reporting Person.

Remarks: Judith A. Boyle is signing on behalf of the Reporting Person pursuant to a Power of Attorney attached hereto as an Exhibit. mahoney2012poa.txt

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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