We have studied the optical properties of thin films of substituted polyphenylene ethynylene/polyphenylene vinylene copolymers. Two novel materials of this type were investigated: poly[ 1,4-phenyleneethylene- 1,4-(2,5-didodecyloxylphenylene)- 1,4-phenyleneethene-1,2-diyl-1,4-(2,5-didodecyloxyphenylene)ethene-1,2-diyl] (DE15) and poly[ 1,4-phenyleneethylene- 1,4-(2,5-di(2-ethylhexyloxy)phenylene)-1,4-phenyleneethene-1,2-diyl-1,4-(2,5-di(2-ethylhexyloxy)phenylene)ethene-1,2-diyl] (DE42). Both pure and fullerene-doped films were prepared by spin coating onto silicon substrates. Their optical properties were obtained by determination of the dielectric function by means of spectroscopic ellipsometry. A large anisotropy of the dielectric function was found both for the pure materials and for their composites with 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C-61 (PCBM) with the optical axis normal to the film surface. The origin of this phenomenon and the influence of the anisotropy on the performance of plastic solar cells are discussed. The optical properties of such DE15/PCBM and DE42/PCBM composites are successfully described in terms of the Bruggemann effective medium approximation. (C) 2003 Elsevier B.V. All rights reserved.