25 mol% Y3+-doped BaCeO3 (BCY25) showed an extremely low activation energy of 0.3 eV for proton conduction at the surface. The resulting overall conductivity at the surface reached 8.24 x 10(-3) S cm(-1) at 400degreesC, which was 3, 8, and 28 times higher than those in the bulk of BCY25, 20 mol% Sm3+-doped ceria, and 8 mol% yttria-stabilized zirconia, respectively. Such fast proton conduction enabled an air/fuel (A/F) sensor using BCY25 as the solid electrolyte to work above 150degreesC for H-2 and above 250degreesC for C2H4. (C) 2004 Kluwer Academic Publishers.