The capacitance of Pt/YSZ air cathodes was measured from the slope of decay curve as a function of contact area and three-phase boundary (TPB) length. The latter two geometrical factors were calculated by image analysis of the micrographs of electrode-electrolyte interface which were taken through a transparent YSZ single crystal used as the electrolyte. It was found that the capacitance is directly proportional to the contact area, but not to the TPB length, suggesting that the whole contact area is covered with adsorbed oxygen. The capacitance per actual contact area is 1300 muF/CM2. This large capacitance was explained as the pseudocapacitance due to a monolayer of adsorbed oxygen.