Barium oxide (BaO) was suggested as a scavenger material to mitigate the harmful siliceous intergranular phase in gadolinia-doped ceria (GDC). The addition of 1-5 mol % BaO to GDC specimens containing 500 ppm SiO2 enhanced the grain-boundary conduction by similar to 250 times. Transmission electron microscopy and electron probe microanalysis attributed this enhancement to the scavenging of the siliceous phase by the Ba-rich phase. In contrast, the grain-boundary conduction was significantly deteriorated when doped with 0.1-0.2 mol % BaO, which was attributed to the dissolution of BaO into the intergranular phase and the consequent increase in the ion-blocking effect at the grain boundary.