Oxygen potential data of U0.85Am0.15O2−x were measured at 1473, 1573, and 1673 K by thermogravimetry. In U1−yAnyO2−x, where An stands for Pu or Am, and for a given value of “y” and Oxygen/Metal ratio, the oxygen potential of U1−yAmyO2−x is higher than that of U1−yPuyO2−x. The valence of cations in the hypostoichiometric region is similar to that of Nd-doped UO2. At the stoichiometric composition, it is estimated to be Am3+, U4+, and U5+ (for charge compensation of Am3+) in the stoichiometric composition. The experimental data were analyzed using a defect chemistry model, and a relationship connecting the oxygen-to-metal ratio, the temperature, and the equilibrium oxygen partial pressure was proposed.