The photooxidation of two poly(ethylene oxide) samples (PEO1w M = 10(5) and PEOhw M = 4 x 10(6)) has been studied under irradiation at long wavelengths (lambda greater than or equal to 300 nm, 35 degreesC) and in the presence of oxygen. Termooxidation experiments, at 50 degreesC, of PEO samples are also reported and compared to photooxidation results. Differences in the ratio ester/formates are observed. Thermooxidation produces equal amounts of esters and formates whereas photooxidation generates formates in a ratio of 5/1 against chain eaters. The oxidation kinetic curves were plotted for films of thickness less than 40 mum to avoid heterogeneous degradation. In order to identify the oxidation products, photo- and thermooxidised films have been submitted to chemical treatments. A mechanism accounting for the main routes of oxidation is proposed. The differences between thermooxidation and photooxidation are explained on the basis of different evolutions of the macroperoxyradicals POO'. The formation of hydroperoxides is favoured in thermooxidation whereas recombination of the peroxyradicals is more likely to occur in photooxidation.