The Bi2O3-Y2O3-CeO2 solid solution with oxygen-deficient fluorite structure is shown to have an oxide-ion conductivity sigma(0) = 0.1 S cm(-1) at 600 degrees C without degradation after annealing at 650 degrees C for over 300 h. The oxygen-deficient perovskite La0.6Sr0.4Co0.2Fe0.8O3-delta proves to be an excellent cathode for an air or O-2 oxidant. With an appropriate strategy for the anode, this electrolyte may be used in a solid oxide fuel cell.