Solid solutions Ce1-xRExO2-δ (RE=Eu, Tb) were prepared by a high-temperature and -pressure method. The products were characterized by Xray diffraction (XRD), TG. Electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and Mossbauer spectroscopy. XRD data analysis showed that all solid solutions crystallized In a single phase cubic fluorite structure. The nonlinear relationships between the lattice parameter and dopant content for both series of solutions were ascribed to the results of cation substitutions and variations of the relative content of oxygen vacancy Vo and defect associations {RE'oceVo} and {Ce'ceVo}. EPR and XPS measurements confirmed the presence of Ce3+ ions in the sogd solutions. For the solid solutions Ce1-xEuxO2-δ, alI Eu ions were determined to be trivalent by XPS and 151Eu Mossbauer measurements. For the solutions Ce1-xTbxO2-δ. all Tb ions were also stabilized in the trivalent state. This result is different from that of the counterpart by hydrothermal conditions, in which a mixed valence of Tb3+/Tb4+ prevails at a higher dopant content. The prepared solutions Cel+xTbxO2-δ were metastable. With increasing temperature, they would he destabilized and decompose into two fluorite phases, accompanied by partial oxidation from Tb3+ to Tb4+. Alternating current impedance spectroscopy showed primarily bulk conduction for all samples. For the solutions Ce1-xEuxO2-δ. the temperature dependence of the ionic conductivity was linear within the temperature range measured with activation energies of 1.05. 0.82, and 0.87 for x=0.2, 0.38. and 0.5, respectively. For the decomposition product of the solid solution Ce0.71 Tb0.29O2-δ. the conductivity gave two linear regions with smaller activation energies; i.e., the activation energy was 0.60 eV below 600℃ and 0.39 eV above 600℃. The higher ionic conductivity (1.1xl02S/cmat720℃) for the decomposition phases of the solution Ce0.71Tb0.29O2-δ was ascribed to an electronic component involved in relation to the presence of the mixed valence of Tb3+/Tb4+ and Ce3+/Ce4+.