The solid solution Ce0.8Sm0.2O2-delta (20CSO) was synthesized by freeze-drying precursor procedure. Well-crystallized powders with nanometric grain sizes were obtained after calcining the precursor at 375 degrees C for 4 h. The effect of Si02-addition and sintering temperature on the properties of the bulk and grain boundary processes were studied. For this purpose, 20CSO-SiO2 samples were prepared by the addition of 0.05 or 0.5 mol% SiO2 to Ce0.8Sm0.2O2-delta, in the form of tetraethyl orthosilicate (TEOS). Also, 2 mol% Co was added to some of the precalcined compositions with and without silica-addition. Cobalt free samples were sintered at 1400, 1500 and 1600 degrees C and cobalt-added samples were sintered 1150 degrees C, for 10 h to obtain dense pellets. The electrical behaviour of the bulk was revealed to be nearly independent on sintering temperature and/or on the addition of impurities of SiO2 and Co to the grain boundaries. This was explained by the low solubility of impurities in the grain fluorite structure. However, the grain boundary resistance showed important differences as function of sintering temperature and with the presence of impurities. The analysis of grain boundary properties suggests that segregated impurities affect the microstructure and also segregation of Sm at the space charge layer, thus changing both the specific grain boundary conductivity and microstructural parameters. (C) 2011 Elsevier B.V. All rights reserved.