In this contribution Differential Scanning Calorimetry (DSC), UV-Vis absorption, optical microscopy and photovoltaic characterizations are used to study the influence of polymer purification on the thermal properties of composite films of poly-3-hexylthiophene, (P3HT) and methanofullerene ([6,6]-phenyl C-61 butyric acid methyl ester) (PCBM). DSC analysis evidence that the composites prepared with commercial polymer, used as received, or after purification, have quite different thermal characteristics. These features, together with the thermal annealing effects on the absorption spectra and optical microscopy, and the photovoltaic properties, indicate that the purification leads to a composite material where the polymer shows improved capability to self-organize. (c) 2005 Elsevier B.V All rights reserved.