Abstract The effect of water vapor addition on the electrochemical oxygen reduction was investigated for the interfaces between fluorite-type oxide-ion conductors (YSZ, ceria) and various types of electrodes at the temperature range 750–1100 K in air. The interface conductivity of YSZ electrolyte/gold electrode at open circuit condition increased when a slight amount of water vapor was added to air at T = 865 K. However, the interface conductivity of YSZ/platinum decreased during a long-term exposure in humid atmospheres. The improvement was correlated to the enhancement of oxygen exchange through water molecules, and the degradation is probably due to the blocking of oxygen molecule migration by the adsorbed water molecules.