Poly(ethylene terephthalate) has been irradiated with UV light of different wavelengths and in various atmospheres. The extent of degradation was monitored by measuring the tensile strength, molecular weight, carboxylic acid endgroups, and fluorescence emission of the polymer. The importance of wavelengths <315 nm in causing deterioration was demonstrated to be mainly due to strong surface absorption which results in surface crazing and ultimate fracture under stress. Irradiation in nitrogen and under vacuum were found to give similar results, but with oxygen present in the system several significant differences were observed. In nonoxidative irradiations, crosslinking and discoloration of the polymer occurred. Under oxidative conditions, chain scissions and fluorescent material build-up resulted, whereas no crosslinking and only slight discoloration was observed. The possibility of a photo-oxidation reaction has thus been suggested, involving hydroperoxide formation, to explain the discrepancy in results obtained for the two types of environments.