We report the electrical and optical properties of molecular films made of homogeneous and segregated mixtures of polythiophene with both C(60) and C(60)(OH)(24-28) compounds, and relate the trends observed with theoretical predictions based on systematic studies of the electronic structure and energetics of polyhydroxylated fullerenes. The electric and optical properties are strongly correlated to the degree of phase segregation in the molecular films and also to the tendency of the hydroxyl groups to cluster on the C(60) cage, thus providing useful information on the potential of hydroxylated fullerenes for the development of buffer layers for fullerene/ conductive polymer solar cells. Finally, we underline the importance of quantifying the changes in the electronic structure of the C(60) Molecules when the chemical functionalization of their surfaces is performed in order to better understand and improve the efficiency of fullerene-based solar cell materials. (c) 2004 Elsevier B.V. All rights reserved.