Blends were obtained by impregnation of an insulating polymer matrix, poly(vinyl chloride), with an oxidant, FeCl3, followed by exposure to pyrrole vapor. The redox properties of films of the blends obtained by this method were studied by cyclic voltammetry and impedance spectroscopy. The results show that thin films (20 mum) are more electroactive than thick films (100 mum) although the proportion of polypyrrole is the same for both (ca. 38%). This electroactive behavior is in agreement with morphology observations, which show different phase separation mechanisms as a function of the film thickness : a spinodal mechanism for the thin film and a binodal mechanism for the thick film.