Photoirradiation of thin films of poly(4-trimethylsilylmethylstyrene) (PTMSMS), poly(4-methylstyrene) (P4MS), and polystyrene (PS) at 254 nm with a low-pressure Hg lamp in air made the surfaces hydrophilic through oxygenation. The hydrophilicity estimated from the contact angle with water was in the order of PTMSMS approximate to P4MS >PS. Formation of carboxylic acid group on the surface and crosslinking in the bulk were demonstrated for PTMSMS, whereas the photoirradiation of PS and P3MS was accompanied by degradation of the polymer main chain. The chemical behaviour of the polymers reflected selectivities in the cleavage of the benzylic C-Si or C-H bond of the 4-substituents toward that of the benzylic C-H bond on the main chain.