The electropolymerization of pyrrole on platinum electrodes has been studied by ac voltammetric and impedance techniques. Anionic species such as chloride, bathophenanthroline disulfonate and ferricyanide, when present in solution during polymerization, adsorbed on the electrode surfaces and increased the rate of the electropolymerization process. The impedance responses of pyrrole solutions were also affected by the applied potential. These anionic species also became incorporated in the polypyrrole film as a result of chemical adsorption and electrostatic interaction.