This paper is devoted to the study of the photo- and thermal-ageing of MDMO-poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene (PPV) blended with methano-fullerene[6,6]-phenyl C-61-butyric acid methyl ester ([60] PCBM), a commonly used active layer of organic solar cells. Thin films of MDMO-PPV, PCBM and MDMO-PPV:PCBM were submitted to photo- and thermal ageing in presence or in absence of oxygen. The modifications of the chemical structure of the materials were analysed by UV-visible and IR spectroscopies. Studying the degradation of pristine materials (MDMO-PPV and PCBM) permit the identification of the markers for characterizing the degradation of each component of the active layer. The degradation of MDMO-PPV was shown to involve a radicalar oxidation implying at first the ethers functions and secondly the double bonds; the photo-oxidation of PCBM was shown to involve the oxidation Of its C-60 moiety. Photo-oxidation of MDMO-PPV:PCBM blends involved the degradation of both components and the presence of PCBM was observed to largely reduce the rate of degradation of MDMO-PPV in comparison with pristine MDMO-PPV. On a kinetic point of view, the same conclusion was obtained for thermo-oxidation and photolysis (irradiation in absence of oxygen). Our experimental results suggest that the decrease of the rate of degradation of MDMO-PPV in blended films during the three kind of ageing (hv, T, O-2) is mainly related to the radical scavenging properties of the PCBM. (c) 2006 Elsevier B.V. All rights reserved.