Ce0.8Gd0.2O2-delta (CGO20) ceramic has been considered as one of the most promising electrolytes for intermediate temperature (IT) fuel cells. It has been reported that some transition metal oxides (TMO), such as MnO2, Fe2O3 and Co3O4, are effective sintering aids for the densification of ceria-based electrolytes. However, very little information is available regarding the effect of TMO addition on the electrical properties of Si-containing CGO20 ceramics. In this study, 0.5 at.% (atomic percent) TMOs have been loaded into the CGO20 with similar to 30 ppm (pure) and similar to 200 ppm (impure) SiO2, respectively, and the sintering behavior and ionic conductivity have been investigated. It has been found that Co3O4 is the most effective sintering aid, and the sintering temperature of CGO20 can be reduced by over 200 degreesC by adding 0.5 at.% CoO4/3. All the TMOs used show a slight effect on the grain-boundary (GB) conductivity of the pure CGO20. However, SiO2 additions, together with MnO2 and Co3O4, have found to be extremely detrimental to the GB conduction of CGO20. As compared to the impure CGO20 the GB resistance has been increased by over six times for the impure one with addition of only 0.5 at.% CoO4/3. On the other hand, in contrast to the Mn or Co doping case, small addition of Fe2O3 has a scavenging effect on SiO2 impurity, and significantly improves the GB conduction of the impure CGO20. Therefore, Fe2O3 could be used as an effective sintering aid, as well as a grain-boundary scavenger for SiO2 contaminated ceria-based electrolytes. (C) 2004 Elsevier B.V. All rights reserved.