The oxygen nonstoichiometry (delta) of Ce0.8PrxTb0.2-xO2-delta (x=0, 0.05, 0.10, 0.15, 0.20) was measured as a function of P-O2 at temperatures between 600 and 900 degrees C by coulometric titration and thermogravimetry. A nonideal solution model, allowing for a linear delta dependence of the partial molar enthalpy of reduction in the dopants, could successfully reproduce the experimentally determined oxygen nonstoichiometry. X-ray absorption near-edge spectroscopy measurements were performed at the Ce/Pr/Tb L3 and L2 edges. The valence state of each dopant was affected by the presence of the co-dopant. The redox properties strongly depended on the lattice strain energy and the mean metal-oxygen bond strength. The thermal and chemical expansion coefficients were determined by dilatometry. The strongly nonlinear behavior of the thermal expansion coefficient originated from the chemical strain due to increasing oxygen nonstoichiometry with increasing temperature.