Generally, the electrochemical oxidation of pyrrole results in polypyrrole film in its doped conducting form. In this paper, it is shown that, in the presence of weak-acidic anions, pyrrole electro-oxidation leads to the formation of a very thin nonconducting overoxidized polypyrrole film. This phenomenon is due to the release of protons at the electrode/electrolyte interface during pyrrole oxidation. XPS analyses confirm (i) the film does not contain doping anions and (ii) the presence of carbonyl bonds; both features are characteristic of overoxidized polypyrrole. (c) 2007 The Electrochemical Society.