Characteristics, such as lattice parameter, theoretical densities, thermal expansion, mechanical properties, microstructure, and ionic conductivities, of Ce0.8M0.2O2-delta (M = Y, Gd Sm, Nd, La) ceramics prepared by coprecipitation were systematically investigated in this paper. The results revealed that the lattice parameter and density based on the oxygen vacancy radius generally agreed with experimental results. Ce0.8Sm0.2O2-delta ceramic sintered at 1500 C for 5 h possessed the maximum ionic conductivity, sigma(800 degrees C) = 6.54 x 10(-2) S cm(-1), with minimum activation energy, E-a = 0.7443 eV, among Ce0.8M0.2O2-delta (M = Y, Gd, Sm, Nd, La) ceramics. The thermal expansion coefficients of Ce0.8M0.2O2-delta (M = Y, Gd, Sm, Nd, La) were in the range of 15.176-15.571 ppm/degrees C, which indicates that the rare-earth oxide dopants have insignificant influence on the thermal expansion property. Trivalent, rare-earth oxide doped ceria ceramics revealed high fracture toughness, with the fracture toughness in the range of 6.393-7.003 MPa m(1/2). According to SEM observation, the cracks are limited to one grain diameter; therefore, the high fracture toughness of rare-earth oxide doped ceria may be due to the toughness mechanism of crack deflection at the grain boundary. Based on the results of grain size and mechanical properties, one may conclude that there is no significant dependence of fracture toughness and microhardness for Ce0.8M0.2O2-delta ceramics on grain size. Correlation between the grain size of Ce0.8M0.2O2-delta ceramics and the dopant species can be explained on the basis of the concept of the rate of grain growth being proportional to the boundary mobility M-b. This leads to a conclusion that the diffusion coefficient of La in Ce0.8La0.2O2-delta>Nd in Ce0.8Nd0.2O2-delta>Sm in Ce0.8Sm0.2O2-delta>Gd inCe(0.8)Gd(0.2)O(2-delta)>Y in Ce0.8Y0.2O2-delta. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.