Ceria co-doped with Sm3+ and Nd3+ powders are successfully synthesized by citric acid-nitrate low-temperature combustion process. In order to optimize the electrical properties of the series of ceria co-doped with Sm3+ and Nd3+, the effects of co-doping, doping content and sintering conditions on grain and grain boundary conductivity are investigated in detail. For the series of Ce-0.9(SmxNd1-x)(0.1)O-1.95 (x=0, 0.5, 1) and Ce1-x(Sm0.5Nd0.5)(x)O-delta (x=0.05, 0.10, 0.15, 0.20) sintered under the same condition, Ce-0.9(Sm0.5Nd0.5)(0.1)O-1.95 exhibits both higher grain and grain boundary conductivity. Compared with Ce0.9Gd0.1O1.95 and Ce0.8Sm0.2O1.9, Ce-0.9(Sm0.5Nd0.5)(0.1)O-1.95 sintered at 1350-1400 degrees C shows higher total conductivity with the value of 1.0 x 10(-2) S cm(-1) at 550 degrees C. In addition, it can be found the trends of grain and grain boundary activation energies of Ce1-x(Sm0.5Nd0.5)(x)O-delta are both consistent with those of Ce1-xNdxO delta, but different from those of Ce1-xSmxO delta, which can be explained as: the local ordering of oxygen vacancies maybe occurs more easily in Nd-doped ceria than in Sm-doped ceria: the segregation amount of Sm3+ is more than that of Nd3+ to the grain boundaries in ceria co-doped with Sm3+ and Nd3+, which is confirmed by X-ray photoelectron spectroscopy (XPS). (C) 2009 Elsevier B.V. All rights reserved.