This article describes a new approach to improve the analysis of the chemical modifications that result from the degradation of polymers under UV-visible light exposure in the presence of oxygen. The tool which is used consists of an irradiation chamber whose atmosphere is composed of O-18(2). The O-18(2) pressure inside the chamber and the hygrometry can be adjusted. In this study, particular attention was paid to the photo-oxidation of poly[2-methoxy-5 -(3',7'-dimethyloctyloxy)- 1,4-phenylenevinylene] (MDMO-PPV). Using infrared spectroscopy, irradiation under the oxygen 18 atmosphere allowed discrimination between compounds formed via fixation of oxygen from the atmosphere and those formed by reorganisation of the matrix. In addition, irradiation of MDMO-PPV in an O-18(2) wet atmosphere allowed for detection of the presence of phenyl formate and aromatic ketone functions. This permitted validating the mechanism of photo-oxidation previously proposed. Additionally, with regard to blends made with MDMO-PPV and methano-fullerene[6,6]-phenyl C-61-butyric acid methyl ester ([60] PCBM), it was demonstrated that this technique facilitates the identification of the photo-oxidation products formed. (C) 2008 Published by Elsevier Ltd.