Rare earth or alkaline earth oxide-substituted ceria (CeO2) powders were successfully prepared via hydrothermal synthetic route. Using these powders after freeze-drying, highly dense ceramics of Ce(1-x)M(x)O(2) with M = Ca2+, Sr2+, Sm3+, Gd3+ and Y3+ and x = 0.2 were prepared by sintering at much lower temperatures than 1700 degrees C, common for CeO2 preparation by conventional ceramic techniques. For example, 20 mol% samaria-substituted ceria with 98% of the theoretical density was sintered at 1450 degrees C. The conductivities determined by an ac impedance method were approximately 100 times higher than that of yttria-stabilized zirconia (YSZ). The conductivity was optimal with Sm3+ substitution (sigma((210 degrees C)) similar to 10(-6) S/cm). The analysis of the impedance spectra indicate that the observed impedance at the relatively low temperatures of measurement (300-600 degrees C) is predominantly dependent on gain boundary effects.