A regular oscillatory behavior was observed during the electropolymerization of pyrrole on an iron electrode in aqueous acetonitrile containing tetrabutylammonium hexafluorophosphate. The periodicity, the amplitude, and the shape of the oscillations depend on the concentrations of the electrolyte species. In order to elucidate the mechanism of this oscillatory phenomenon, the ring-disk-electrode technique was used. A mechanism involving a catalytic oxidation of pyrrole by Fe3+ ions produced during iron oxidation is proposed and discussed.