This study examined the effect of adding SrO on the grain-boundary conduction of Ce0.9Gd0.1O1.95 (gadolinia-doped ceria) containing 500 ppm SiO2. The apparent grain-boundary resistivity at 300 degrees C decreased drastically from 746.7 to 0.90-1.97 k Omega cm upon doping with >= 1 mol % SrO, while the grain-interior resistivity increased gradually from 3.1 to 11.6 k Omega cm as the SrO concentration was increased up to 5 mol %. Therefore, doping with 1 mol % SrO resulted in the minimum total resistivity. The electron probe X-ray microanalysis and the analysis of the lattice parameters suggest that the 140-500-fold enhancement in the grain-boundary conduction is attributed to the scavenging of the highly resistive siliceous phase by the SrO-containing phase. (C) 2008 The Electrochemical Society.