Both doped zirconia and ceria have been widely recognized as promising electrolytes in solid oxide fuel cells (SOFC). Total conductivity is an important parameter to evaluate solid electrolytes. It is well know that the contribution to the total conductivity by grain boundaries is especially pronounced for SiO2-contaminated electrolytes. In this study, we report on the different conduction behaviors of grain boundaries (GB) found in SiO2-contaming (impure) 8YSZ (8 mol% Y2O3-doped ZrO2) and CGO20 (10 mol% Gd2O3-doped CeO2) ceramics. In the grain size range (similar to 0.5-10 mu m) studied, the GB conductivity of impure CGO20 ceramics constantly decreases with increasing grain size, in contrast to that observed in impure 8YSZ electrolytes whose GB conductivity increases almost linearly with grain size. It is also found that the variation in GB conductivity versus grain size is different from case to case, depending on the sintering/annealing conditions used to fabricate the ceramics. Two mechanisms were proposed to explain the GB behaviors of the impure 8YSZ and CGO20 ceramics. For doped ceria, the GB phases are supposed to be inert, which do not react with or dissolve into the matrix. Increasing sintering temperature leads to not only grain growth but also change in viscosity and wetting nature of the GB phases. These two factors promote further propagation of the GB phases along the grain boundaries, leading to an increased GB coverage fraction. For doped zirconia, however, the major factor dominating the GB conduction is the further dissolution of SiO2 into zirconia lattice as a result of increase in sintering temperature or/and time. In addition, we will also evaluate and discuss the validities of the three models that are widely used to analyze the GB conduction in solid electrolytes. (c) 2006 Elsevier B.V All rights reserved.