X0303 4 2009-03-23 0001097864 ON SEMICONDUCTOR CORP ONNN 0001368095 Mahoney Robert Charles 5005 EAST MCDOWELL ROAD PHOENIX AZ 85008 1 Exec VP Sales & Marketing Common 2009-03-23 4 F 0 2080 4.61 D 59228 D On March 23, 2009, the Reporting Person vested in 6,666 restricted stock units ("RSUs") and as a result acquired 6,666 shares of common stock of the Issuer pursuant to the Reporting Person's June 5, 2006 grant of RSUs. The RSUs acquisition was previously reported on an earlier Form 4 filed by the Reporting Person. This Form 4 reports the disposition of shares withheld by the Issuer pursuant to the RSUs' grant agreement in order to satisfy the Reporting Person's withholding obligations for various taxes due upon the vesting of the RSUs. The Issuer will pay the taxes on behalf of the Reporting Person. Judith A. Boyle is signing on behalf of the Reporting Person pursuant to a Power of Attorney attached herein as an Exhibit. mahoney2009poa.TXT Judith A. Boyle, Attorney-in-Fact 2009-03-24