The structure, the thermal expansion coefficient, ionic and electronic conductivities of Ce0.8Gd0.2-xMxO2-delta (for M: Sm, x = 0-0.1, and for M: Bi, La, and Nd, x = 0.05) solid solutions, prepared for the first time hydrothermally, are investigated. The uniformly small particle size (23-65 nm) of the materials allows sintering of the samples into highly dense ceramic pellets at 1300-1400 degrees C, significantly lower temperature, compared to that at 1600-1650 degrees C required for ceria solid electrolytes prepared by solid state techniques. The maximum conductivity, sigma(700 degrees C) similar to 6.50 x 10(-2) S cm(-1), E-a = 0.59 eV, is found at x = 0.1 for Sm-codoping. Among various metal co-dopings for x = 0.05, the maximum conductivity is found for Sm-co-doping (sigma(700 degrees C) similar to 5.13 x 10(-2) S cm(-1), with E-a = 0.65 eV). The electrolytic domain boundary (EDB) of Ce0.8Gd0.1Bi0.1O2-delta is found to be 1.2 x 10(-19) atm, which is relatively lower than that of singly doped samples. The thermal expansion coefficients, determined from high-temperature X-ray data are 11.7 +/- 0.6 x 10(-6) K-1 for the CeO2, 12.2 +/- 0.6 x 10(-1) K-1 for Ce0.8Gd0.2O2-delta, and increase with co-doping to 13.07 +/- 0.6 x 10(-6) K-1 for Ce0.8Gd0.15Sm0.05O2-delta. These results suggest that co-doping can further improve the electrical performance of ceria-based electrolytes. (C) 2009 Elsevier B.V. All rights reserved.