When irradiated at >400 nm in air, pendant benzil groups of 1-phenyl-2-(4-vinylphenyl)-1,2-ethanedione/styrene (VBz/S) copolymer films are transformed almost quantitatively into benzoyl peroxide (BP) groups. Subsequent heating at 91 degreesC converts the pendant benzoyl peroxide groups to esters and benzoic acid moieties, and there is significantly more cross-linking than main-chain cleavage. Irradiation of the VBz/S copolymer films at 366, 313, and 254 nm also results in formation of BP groups, but they are transformed in situ upon absorption of a second photon by the matrix. The ratios of the relative rate constants for BP formation and subsequent transformation upon absorption by a second photon decrease with decreasing wavelengths of radiation. Irradiation of a film composed of a nonmiscible intimate mixture of poly(1-phenyl-2-(4-vinylphenyl)-1,2-ethanedione) (PVBz) and polystyrene (PS) at >400 nm in air does not lead to discernible BP concentrations as well. Instead, the unreacted pendant benzil groups act as photosensitizers to transform the peroxy moieties almost immediately. In addition, we demonstrate that cross-linking of the VBz/S copolymer film, induced by 254 run radiation, can be utilized to record a negative image.