Samples of Sm and Mg co-doped ceria electrolytes of Ce(0.8)Sm(0.2-x)Mg(x)O(2-delta) (x = 0, 0.05, 0.10, 0.15 and 0.20) were sintered from powders obtained by solid-state reaction method. The structures and electrical properties were characterized by X-ray diffraction (XRD) and AC impedance spectroscopy (IS). The thermal expansion curves of samples were measured and the thermal expansion coefficients between 100 and 800 degrees C were calculated. Results Showed that co-doping with appropriate ratio of Sm and Mg can improve the electrical performance of ceria-based electrolytes. As the substitution amount of Mg for Sm increased up to 50 mol%, the conductivity Of the samples maintained almost unchanged or even slightly higher than that doped only with Sm in intermediate temperature. The results Suggest that the materials cost for producing the ceria-based intermediate temperature solid oxide fuel cells (IT-SOFCs) may be significantly reduced owing to the cost difference between Mg and Sm. (C) 2008 Elsevier Ltd. All rights reserved.