Abstract Modified electrodes were obtained by electrodeposition on Pt of polypyrrole (PP) from aqueous solutions. The electrodes obtained in the absence of cytochrome c were tested by cyclic voltammetry in aqueous solutions containing different supporting electrolytes. The current corresponding to the oxidation and reduction of PP as well as the peak potentials depends, when using salts of the same cation, on the nature of the anion. The different were interpreted in terms of the anion flux in and out the film and/or its adsorption on the film surface which accompanies the oxidation and the reduction of the PP, respectively. The change of the cation also produces some effect on the electrochemical film behavior. The modified film behaves like a metallic electrode, as it concerns the cv of electroactive species in the solution, when the PP is in the oxidized form. If the electrode is exposed to very positive potentials it cannot be reduced further, but shows selectivity as only some molecules can be reduced or oxidized to the electrode. In the presence of incorporated cytochrome c, two new, well defined, oxidation and reduction peaks are visible in cv. The peaks became more and more evident after successive cyclic scans.