KrF laser irradiation of thin films of poly(4-trimethylsilylmethylstyrene) (PTMSMS) under atmospheric conditions induced oxygenation at the film surface and cross-linking inside the film. The irradiated film surface became hydrophilic owing to the formation of carboxylic acid group, and the photo-cross-linking caused the insolubilization of the film inside. Similar irradiation of poly(4-methylstyrene) (P4MS) and polystyrene (PS) films, on the other hand, induced oxidative degradation of the polymer main chain. Selectivity in the bond cleavage involved in their photoreactions would lead to the contrast results. Selective cleavage of the benzylic C-Si bond of PTMSMS gives benzyl-type radicals, which react with oxygen, leading to the formation of carboxylic acid groups at the surface, or couple with each other, giving the cross-linked structure inside the film. Cleavage of the C-H bond of the main chain in P4MS and PS would be involved in the oxidative degradation of the polymer main chain. Since the cross-linking is an apparent two-photon reaction and requires the closely formed radicals, efficiency of the C-Si bond cleavage and the following radical formation at the 4-position of the benzene ring also must play a crucial role in the cross-linking ability.