Freeze-drying was used to prepare nanocrystalline powders of Ce1-xSmxO2-delta, x = 0.05-0.3, with high-purity and crystallite sizes in the range 10-15 nm. These powders were used to obtain ceramic samples with 4-7 mu m in average diameter by firing at 1873 K, without sintering additive, and < 1 mu m for samples sintered at 1423 K, with previous addition of cobalt nitrate solution to the freeze-dried powders. Impedance spectroscopy was used to deconvolute components of the spectra related to grain interiors and grain boundaries. For samples with grain sizes in the range 4-7 mu m, the bulk conductivity increases with decreasing content of Sm up to 10%, whereas the grain boundary conductivity shows the opposite trend. However, samples with relatively low content of Sm and submicrometer grain sizes, obtained with addition of Co, retain the highest conductivity for both the bulk and grain boundary conductivities. The differences in grain boundary conductivities between samples with small and large grain sizes were about 1 order of magnitude for 20% Sm and 2 orders of magnitude for 5% Sm and 10% Sm. Differences in grain boundary behavior were interpreted on the basis of a space-charge layer adjacent to the boundary core. The estimated values of grain boundary thickness were in the order of 1 nm for samples prepared with and without addition of cobalt nitrate. (c) 2006 The Electrochemical Society.