The effects of water addition to an oxidant on the cathodic overpotential of La-doped BaCoO3 and Sm0.5Sr0.5CoO3 cathodes were investigated in this study. It was found that the overpotential of La-doped BaCoO3 decreased by the addition of H2O to oxygen. In particular, the decrease in the overpotential by addition of H2O was greater with the decreasing operating temperature of the cell. Therefore, it becomes clear that the maximum power density increased by the addition of H2O to the oxidant. Positive effects of water on the cathodic overpotential are also observed on Sm(0.5)Sr(0.)5CoO(3). Therefore, the positive effects of water on cathodic reaction are not limited to La-doped BaCoO3, but are also expected on general oxide cathodes. The positive effects of a humidified oxidant were sustained stably over 50 h, and X-ray diffraction measurement suggests that there is no change in the crystal structure of an La-doped BaCoO3 cathode or an LaGaO3-based oxide electrolyte. The improved cathodic activity by addition of H2O was studied further by using O-18 surface exchange measurement. Although the diffusion coefficient was independent of the coexisting water, the surface exchange coefficient was greatly increased by the addition of water. Therefore, water existing in a cathode seems to assist the dissociation of oxygen into an oxide ion. Therefore, it is considered that a humidified oxidant is one of the effective methods to suppress the cathodic overpotential at decreased temperature. (c) 2005 The Electrochemical Society.