The structure, the thermal expansion coefficient, electrical conductivities of Ce0.8Gd0.2-xMxO2-delta (for M: Bi, x=0-0.1, and for M: Sm, La, and Nd, x=0.02) solid solutions, prepared for the first time hydrothermally, are investigated. The uniformly small particle size (28-59 nm) of the materials allows sintering of the samples into highly dense ceramic pellets at 1300-1400 degrees C. The maximum conductivity, sigma(700 degrees C) around 4.46 x 10(-2) S cm(-1) with E-a =0.52eV, is found at x = 0.1 for Bi-co-doping. Among various metal-co-dopings, for x = 0.02, the maximum conductivity, sigma(700 degrees C) around 2.88 x 10(-2) S cm(-1) with E-a = 0.67 eV, is found for Sm-co-doping. 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 the singly doped samples. The thermal expansion coefficients, determined from high-temperature X-ray data are 11.6 x 10(-6) K-1 for the CeO2, 12.1 x 10(-6) K-1 for Ce0.8Gd0.2O2-delta, and increase with co-doping to 14.2 x 10(-6) K-1 for Ce0.8Gd0.18Bi0.20O2-delta. The maximum power densities for the single cell based on the codoped samples are higher than that of the singly doped sample. These results suggest that co-doping can further improve the electrical performance of ceria-based electrolytes. (C) 2009 Elsevier B.V. All rights reserved.