Poly(styrene-co-acrylonitrile) (SAN) films were exposed to long wavelength radiation (λ ≥ 300 nm) and to short wavelength radiation (λ = 254 nm) in the presence of atmospheric oxygen. Fourier-transform infrared (FTIR) and ultraviolet-visible (UV-Vis) spectroscopies were used to follow the chemical changes in the polymer. The different photoproducts formed were identified by coupling FTIR and UV-Vis analysis to chemical derivatisation reactions or physical treatments such as thermolysis and photolysis. A mechanism accounting for the photooxidation of SAN is then proposed. This mechanism involves both the styrene and the acrylonitrile units. Moreover, the products responsible for the discoloration occurring during SAN photooxidation are shown to result from styrene oxidation.