Although BaZr0.8Y0.2O3-delta (BZY) possesses large bulk proton conductivity and excellent chemical stability, its poor sinterability and grain boundaries block proton conduction. In this work, the effect of Ca as a co-dopant and as a sintering aid (as CaO), on the sinterability, proton conductivity, and fuel cell performance of BZY was investigated. The addition of 4 mol% CaO significantly improved the BZY sinterability: BZY pellets with densities of 92.7% and 97.5% with respect to the theoretical density were obtained after sintering at 1500 degrees C and 1600 degrees C, respectively. The improved BZY sinterability by CaO addition resulted also in a large proton conductivity; at 600 degrees C, the total conductivity of BZY-CaO was 2.14 x 10(-3) S/cm, in wet Ar. Anode-supported fuel cells with 25 mu m-thick BZY-CaO electrolyte membranes were fabricated by a dual-layer co-firing technique. The peak power density of the fuel cell with a BZY-Ni/BZY-4CaO/BZY-LSCF (La0.6Sr0.4Fe0.8 Co0.2O3-delta) configuration was 141 mW/cm(2) at 700 degrees C, several times larger than the reported values of BZY electrolyte membrane fuel cells sintered with the addition of CuO or ZnO, demonstrating promising features for practical fuel cell applications.