Active reaction sites for 02 reduction in La0.~Sr01MnO3 electrode have been characterized by addressing the origin of the cathodic polarization effects on this electrode material. Cathodic polarization (up to -1.2 V vs. Pt reference electrode} had several effects on O2 reduction kinetics. First, the O2 reduction rate was favorably increased when the perovskite electrode was cathodically polarized. Second, in situ x-ray photoelectron spectroscopy results indicated that the Mn ions are electrochemically reduced and concomitantly the oxygen stoichiometry decreases. Reduction of Mn ions was further demonstrated in the cyclic voltammogram traced under nitrogen atmosphere. Third, hysteresis in cathodic currents was observed in the cyclic voltammograms of the perovskite/YSZ/Pt system, and the hysteresis phenomena were more prominent at higher O~ pressure. We interpreted these findings to mean that the internal and/or external surface oxide vacancies participate in the O2 reduction reaction. However, it has been explained from the Po2-dependent hysteresis phenomena that, even though those surface sites are active in the O2 reduction~ their activity is less than that of the three-phase boundary sites since additional diffusional processes are required for the former sites. Consequently, the three-phase boundary sites are the major reaction sites at lower O2 pressure, which leads to a small hysteresis. However, at higher 02 pressure, the surface sites also participate in the reaction, resulting in a larger hysteresis.Strontium-doped lanthanum manganites (Lal xSr= MnO3) are widely studied for their possible applications as cathode materials in solid oxide fuel cell systems. 1-3 Even though many results have been reported on aspects of the oxygen reduction kinetics and on the nature of the active reaction sites, there still remain controversies on this issue in the literatureYThe manganite perovskites are electronic conductors, 9 but under cathodic potential they are supposed to be partially reduced, creating oxide vacancies. 6' 1~ Rate enhancement found in cathodically polarized electrodes has been interpreted with an assumption that surface oxide vacancies are active for oxygen reduction. 5' Another set of the previous results that oxygen reduction kinetics can be improved by Sr doping to LaMnO~ has also been explained with the same assumption. ''6On the contrary, other reports suggested that the surface sites cannot participate in O2 reduction since the O 2-bulk diffusion in the manganites is negligible compared to the cobalt analogs or YSZ electrolytes. 12 The favorable effect of the cathodic polarization was also attributed to the enlargement of the three-phase boundary lines. 7' 13 It is likely, however, that the surface or grain boundary diffusion in porous electrodes cannot totally be discarded for their possible contribution to the overall kinetics since in general surface or grain boundary diffusion is much faster than bulk diffusion.In this study, we tried to elucidate the nature of the active sites for O2 reducti...