The effect of small amounts ( < 5 mol.%) of cobalt oxide on the sintering characteristics and electrochemical performance of cerium oxide solid solutions have been evaluated. Ce0.8Gd0.2O2-x powder with an average crystallite size of 20 nm served as model substance for electrolytes with a high oxygen ionic conductivity and low electronic conductivity. Sintering to > 99% of the theoretical density has been achieved at temperatures below 900 degreesC resulting in an average grain size less than 150 nm in the sintered material. AC and DC conductivity measurements on bulk material revealed that for transition metal oxide doping below 2 mol.% the ionic conductivity remains unchanged compared to pure CGO. (C) 2000 Elsevier Science B.V. All rights reserved.