The electrochemical overoxidation of conducting polypyrrole nitrate film in aqueous solutions was studied by cyclic voltammetry, in-situ spectro-electrochemical measurement and FTIR spectroscopy. The overoxidation, which is entirely irreversible, results in the dedoping of conducting polypyrrole and the formation of carboxyl group. The potentials at which the overoxidation occurs depends on the pH values of the aqueous solutions. The higher the pH value, the lower the overoxidation potential will be, i.e. the easier the occurrence of the overoxidation, which is coincident with the electrolysis reaction of water to give out oxygen. It was supposed that the oxygen produced in the electrolysis of water results in the overoxidation of the conducting polypyrrole films.