A series of ceria-based solid solution electrolytes, CoO and Sm2O3 codoped ceria Ce0.8Sm0.2-xCoxO2-delta (denoted as CSO for x = 0 and CSCx for x = 0.01-0.09), are synthesized via a carbonate co-precipitate method. The phase composition of the precursor is determined by XRD, EA and TG/DSC. The oxidation valence of Co in the samples calcined at different temperatures is probed by XPS and determined to be in +2 valence state at the temperature >1000 degrees C. The sintering investigation demonstrates that the addition of Co promotes densification to occur at lower temperatures with a uniform grain growth. Impedance spectroscopy measurements indicate an improvement in the grain and the grain boundary conductivities for the samples with appropriate Co/Sm ratio. The optimal amount of Co additive is proved to be 7 mol%. It shows that CSC0.07 exhibits higher conductivity (sigma(t) = 0.0538 S cm(-1) at 600 degrees C) and lower activation energy (E-d = 0.567 eV) compared with CSO and other CSCx. The results suggest that cobalt oxide could be used not only as an effective sintering aid but as a favorable dopant to ceria as well. (C) 2012 Elsevier B.V. All rights reserved.