Co-doped ceria-based electrolytes of Ce(1-x)La(x-y)Ca(y)O(2-delta), wherein x = 0.15 and 0.20, 0 <= y <= x, were sintered from powders obtained by solid state reaction method. The phase identification, thermal expansion and ionic conductivities of samples were studied by X-ray diffraction (XRD), dilatometry and AC impedance spectroscopy (IS). Results showed that the samples of co-doping with La and Ca can significantly increase the ionic conductivity and lower activation energies compared with those of the singly doped ones in the temperature range of 500-800 degrees C. The ionic conductivities of co-doping samples decrease with Ca content. Although both systems reached the optimum ionic conductivity at y = 0.05, Ce(0.85)La(0.15-y)Ca(y)O(2-delta) exhibits better electrical performance. The results also showed that all the synthesized samples were fluorite-type ceria-based solid solutions. The thermal expansion was linear for all the samples. (C) 2009 Elsevier Ltd. All rights reserved.