The photodegradation of heterophasic ethylene-propylene copolymers and their homopolymers has been investigated by the study of hydroperoxide formation, and by electron spin resonance (e.s.r.) and infra-red spectroscopy. On irradiation of polymer films in air at 60-degrees-C with a light of wavelength longer than 300 nm, polymer hydroperoxides are formed. The behaviour of peroxyl radicals generated in the samples on irradiation of polymer hydroperoxides with ultraviolet light at -50-degrees-C in vacuum has been identified and compared with a combination of e.s.r. and derivatization reactions. From Fourier transform infra-red spectroscopy the structure of peroxyl radicals formed is identified.