The photo-oxidative degradation of 25-mu-m-thick films of copolymers of acrylonitrile with styrene (SAN), alpha-methylstyrene (AMS:AN) and p-methylstyrene (PMS:AN) by UV-B radiation (maximum intensity at 313 nm) resulted in the formation of conjugated unsaturation, hydroxyl and carboxyl structures, and main-chain scission and crosslinking. Oxidation increased with increasing proportion of the styrenic monomer, and in the order AMS:AN, SAN, PMS:AN. Chain scission was predominant in ASM:AN and SAN, but crosslinking was the main reaction in PMS:AN. The tensile strength and elongation to fracture of PMS:AN were relatively unaffected by high radiation doses, but decreased substantially for SAN and AMS:AN, consistently with the changes in molecular weight.