The photolysis and the photooxidation of poly(ethylene terephthalate, PET)-modified copolymer have been investigated. The copolymer (COP) that results from the polycondensation of terephthalic acid, ethyleneglycol and 1,4-cyclohexanedimethanol was studied and a comparison with the behaviour of PET was made. Under vacuum irradiation at lambda > 300 nm, the pure photolytic processes involve Norrish type I and II reactions. The formation of the hydroxylated and carbonyl containing photolytic products, which involves hydrogen abstraction from the polymeric backbone, is enhanced in the COP: this result is related to the labile hydrogen atom on the tertiary carbon atom of the cyclohexane units. The pure photolytic processes, in the presence of oxygen, induces oxidation of the aliphatic sequences under polychromatic (lambda > 300 nm) and monochromatic irradiations (254 and 365 nm). The tertiary carbon atoms of the cyclohexane units are shown to be more oxidizable than the methylene groups in the alpha-position of the eater groups. The decomposition of the hydroperoxides so formed leads to the formation of acetic and formic acids that can migrate out of the film; it also generates hydroxy radicals that is one path leading to the ortho-hydroxylation of aromatic rings which gives fluorescence emission to the photooxidised films. The penetration of polychromatic light in the COP sample limits the degradation to the 100 mu m top layer. In this superficial layer, chain scission occurs to a large extent as revealed by the identification of terephthalic acid. (C) 2000 Elsevier Science Ltd. All rights reserved.