The ZrO2-TiO2-Y2O3 system exhibits high ionic and electronic conductivity at high temperatures under low oxygen partial pressures. Using this system as a membrane for gas separation, hydrogen can be produced from direct water splitting at high temperatures. Vaporized water was dissociated at high temperatures and oxygen permeated through the membrane by the oxygen partial pressure difference. With increasing oxygen partial pressure difference, water dissociation was promoted and the amount of the produced hydrogen was increased.