In high performance polymer: fullerene bulk heterojunction solar cells the nanoscale morphology of interpenetrating acceptor:donor materials is optimized through appropriate preparation conditions such as annealing and choice of solvent, but this initial state-of-the-art morphology will not remain stable during long-term operation. We report the effects of prolonged storage at elevated temperatures on both the morphology and the photovoltaic performance for the model systems MDMO-PPV:PCBM and P3HT:PCBM as compared to 'High T-g PPV':PCBM based solar cells, where the 'High Tg PPV' is characterized by its high glass transition temperature (138 degrees Q. In situ monitoring of the photocurrent-voltage characteristics at elevated temperatures, in combination with a systematic transmission electron microscopy (TEM) study and complementary optical spectroscopy, reveals distinct degradation kinetics and morphological changes that indicate the occurrence of different underlying physico-chemical mechanisms. (c) 2008 Elsevier B.V. All rights reserved.