The structure, thermal expansion coefficients, ionic and electronic conductivities of Ce1-xLaxO2-delta (x = 0-0.30) solid solutions, prepared for the first time hydrothermally, were investigated. The uniformly small particle size (25-50 nm) of the hydrothermally prepared materials allows sintering of the samples into highly dense ceramic pellets at 1300-1400 degrees C, a significantly lower temperature, compared to that at 1600-1650 degrees C required for ceria solid electrolytes prepared by solid state techniques. The solubility limit of La2O3 in CeO, was determined to be around 20 mol.%. The maximum conductivity, delta(600 degrees C) similar to 6.4 x 10(-3) S/cm with E-a = 0.73 eV, was found at x = 0.15. The thermal expansion coefficients, determined from high-temperature X-ray data are 11.7+/-0.6 X 10(-6) K-1 for the CeO2, and, increase with increasing La substitution (15.0 X 10(-6) K-1 at x = 0.20).