Cobalt doping between 2 and 10 at.% was utilized to lower the required sintering temperature of materials in the series BaCe0.5Zr0.4(Y,Yb)(0.1-y)CoyO3-delta to between 1373 and 1698 K. The required sintering temperature decreased with increasing Co content; however. significant electronic conductivity was observed in both oxidizing and reducing environments for materials with 10 at.% Co. This was accompanied by a loss of chemical stability in H2O/H-2 and CO2 environments. BaCe0.5Zr0.4Yb0.07Co0.03O3-delta was stable in these environments and provided the highest proton conductivity of the materials tested, 1.98 x 10(-3) S/cm at 923 K in humidified H-2. Measurements in a hydrogen concentration cell indicated that the total ionic transference number for this material was between 0.86 and 1.00 with proton transference number between 0.84 and 0.75 at 773 and 973 K respectively. Under oxidizing conditions, the ionic transference number decreased to below 0.10. The grain boundary resistance dominated the total conductivity at low temperatures but was found to decrease with increased sintering temperature due to grain growth. (C) 2008 Elsevier B.V. All rights reserved.