Degradation studies of poly(2-methoxy-5-(3＇,7＇-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV), fullerenes ((6,6)-phenyl C-61-butyric acid methyl ester (PCBM) and C-60), and mixtures, which are the photoactive components in plastic solar cells, are shown. The degradation processes of the individual components and of a 1:3 mixture are characterized by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and by current/voltage (I-V) measurements of devices under the influence of light and oxygen. A faster degradation rate was found for the polymer compared with C-60. In composites with fullerenes, the stability of MDMO-PPV is enhanced due to the fast electron transfer to C-60. (C) 2000 Elsevier Science B.V. All rights reserved.