Novel syntheses of peptidyl ketones and peptidyl diketones on polymer support are described. Peptidyl phosphoranylidene acetates were prepared via C-acylation of polymer-supported phosphorus ylides. Selective alkylation of the ylide carbon with various alkyl halides, such as methyl iodide and benzyl bromide was established. Peptidyl diketones were obtained by oxidative cleavage. Peptidyl ketones were furnished by hydrolysis of the peptidyl phosphorus ylides under either basic or acidic conditions.