Potential oscillations have been observed for the first time during the electro-oxidation of Pb electrode in sulfuric acid with SCN-. The oscillations occur in the phase transition region of PbSO4/PbO2 accompanying periodic oxygen evolution. Raman spectroscopy and X-ray diffraction results show that the PbO2 film formed by electro-oxidation can be reduced to PbSO4 chemically by SCN-. Therefore, the oscillatory phenomena found here could be ascribed to the alternate formation and reduction of PbO2 films on electrode surface, namely, the PbO2 film formed by the electro-oxidation of Pb via PbSO4 can be reduced to PbSO4 again by S2- which comes from the dissociation of SCN-. Oxygen evolution can promote the formation of PbO2 and vice versa. The oscillatory mechanism has been further proved either by the replacement of SCN- with another source of S2- from thiourea or by the recurrence of oscillations on an electrodeposited PbO2 film. (C) 2007 The Electrochemical Society.