Polyaniline (PAN) films were prepared using sequential linear potential sweeps in sulfuric, dimethylbenzenesulfonic, and hydrochloric acids. The films were converted from their reduced form to an oxidized state in several different supporting electrolytes, and a mechanism of oxidation has been proposed, based on cyclic voltammetry experiments at differentpH. Apparent diffusion coefficients for bisulfate, chloride, and dimethylbenzenesulfonate in several PAN films were obtained, using a potential step technique. The results demonstrate that electrical neutrality in PAN films is maintained at a pH below 1 by the egress of hydronium ion, whereas at pH above 2, neutrality is maintained by the insertion of anions into the film.