The oxidation of poly(4-methylstyrene) with cerium(IV) and cobalt(III) complexes in glacial acetic acid introduces functional groups selectively into the 4-methyl position. The polymer backbone methine groups are almost completely protected from oxidation due to stereo-electronic effects. The reaction proceeds by double electron transfer to give intermediate polymeric benzyl cations. These combine with nucleophiles present in the reaction mixture, such as acetic acid, trichloroacetic acid, or nitrate, to give polymeric benzyl acetates, benzyl trichloroacetates, and benzyl nitrates. These substituents can be exchanged for each other, leading to poly(4-methylstyrenes) containing between 10 and 45% of a single reactive leaving group. The effects of reaction conditions on oxidation yields and selectivity are described.