Scandium and yttrium co-doped barium zirconate [BaZr0.85ScxY0.15-xO3-delta (x=0, 0.05, 0.075, 0.10, 0.15)] have been investigated in terms of phase relationship, microstructures, and electrical conductivity. The bulk conductivity of the scandium and yttrium co-doped barium zirconate increased with the dopant ratio of yttria. BaZr0.85Sc0.05Y0.10O3-delta had the highest grain-boundary conductivity among the scandium and yttrium co-doped barium zirconates in this study. But, BaZr0.85Sc0.15O3-delta, BaZr0.85Sc0.10Y0.05O3-delta, BaZr0.85Sc0.75Y0.075O3-delta, and BaZr0.85Sc0.05Y0.10O3-delta consisted of a single cubic perovskite phase at 1600 degrees C and their densities of grain-boundary were smaller than that of BaZr0.85Y0.15O3-delta. From the observation of microstructure and results of grain boundary-conductivity measurement, we can say that yttrium is a dopant that increases specific grain-boundary conductivity and bulk conductivity, and scandium is a dopant that increases the grain size. Thus, there is a trade-off relation between the grain size and specific grain-boundary conductivity based on the mixing ratio of scandia to yttria. The total conductivity of BaZr0.85Sc0.05Y0.10O3-delta at 600 degrees C was estimated to be 1.6x10(-2) S cm(-1), which is the highest-class conductivity among reported trivalent cation-doped barium zirconates. (C) 2008 The Electrochemical Society.