The effect of the addition of copper-doped bismuth-vanadium oxide to yttria-stabilized zirconia (YSZ) on the sinterability and electrical conductivity, especially in reducing atmospheres, has been investigated. When Bi2Cu0.1V0.9O5.35 was used as a sintering aid, the sintering temperature could be reduced to 800 degrees C, whereas the conventional sintering process requires temperatures above 1300 degrees C. Although the conductivity of YSZ with 1.5 mol % Bi2Cu0.1V0.9O5.35 was slightly lower than that of pure YSZ sintered at 1300 degrees C, no Po-2 dependence was observed under reducing conditions (e.g., Po-2 = 10(-5)-10(-2)4 atm), which indicates that the electrolyte is applicable to solid-oxide fuel cells.