In polymer-fullerene organic solar cells, efficient charge generation takes place via photoinduced charge transfer between the photoexcited conjugated polymer and acceptor-type fullerene (C-60) molecules. The bulk transport properties of poly[2-methoxy-5-(2-ethyl)hexoxy-1,4-phenylenevinylene]/fullerene composites have been studied by means of DC measurements and dielectric spectroscopy. The dielectric properties were investigated on a broad frequency range, according to the volume fraction of fullerene in the MEH-PPV film. The static conductivity increased by four orders of magnitude as the fullerene concentration increases to 40 vol.%. The conductivity enhancement was interpreted by the formation of a conducting path for a percolation threshold of 40 vol.%. This was confirmed by admittance spectroscopy showing that the conductance of the composite becomes independent of the frequency for a fullerene concentration higher than 37 vol.%. Interfacial polarization processes have been investigated by capacitance and dielectric loss measurements. The change in dielectric properties could be explained in terms of composite morphology changes, which were characterized by atomic force microscopy (AFM). (c) 2005 Elsevier B.V All rights reserved.