Active reaction sites for O-2 reduction in La0.9Sr0.1MnO3 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 O-2 reduction kinetics. First, the O-2 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-2 pressure. We interpreted these findings to mean that the internal and/or external surface oxide vacancies participate in the O-2 reduction reaction. However, it has been explained from the P-O2-dependent hysteresis phenomena that, even though those surface sites are active in the O-2 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 O-2 pressure, which leads to a small hysteresis. However, at higher O-2 pressure, the surface sites also participate in the reaction, resulting in a larger hysteresis.