Powders of Ce_{0.8}Pr_{x}Tb_{0.2-x}O_{2-δ} solid solutions (x = 0, 0.05, 0.10, 0.15, 0.20) were synthesized by co-precipitation. After sintering at 1500^{o}C in Air for 12h and subsequent annealing in either reducing or oxidizing conditions, the powders were observed by XRD to be single phase fluorites. The oxygen nonstoichiometry was determined over a wide PO2 range at 600/700/800/900^{o}C by coulometric titration and validated for the x = 0.20 composition by thermogravimetry. XANES measurements were performed at the Ce/Pr/Tb L3 and L2 edges. The valence state of each dopant was found to be affected by the presence of the co-dopant. The total 4-valent dopant fraction obtained from XANES, matches well the one determined by coulometric titration. A defect model was developed that accounts well for the determined oxygen nonstoichiometry Ce_{0.8}Pr_{x}Tb_{0.2-x}O_{2-δ} over the entire P_{O2} range.