The oxygen to metal ratio (O/M) is directly related to oxygen potential, which strongly influences the sintering and irradiation performance of nuclear fuels. A better understanding of these two parameters is therefore of major interest. To further ascertain the correlation between O/M ratio and oxygen potential in Am-bearing MOX, several thermodynamic descriptions are being developed. Despite their differences, they all involve the valence of actinide cations (e.g., U, Pu, and Am) as essential parameters. However, as no experimental data on their valence are available, these models rely on assumptions. In the present work, we coupled X-ray diffraction and X-ray absorption spectroscopy to follow the behavior of Pu and Am in three hypo-stoichiometric, U-free Pu1-yAmyO2-x compounds. We provide for the first time a quantitative determination of Pu and Am valences, demonstrating that plutonium reduction from Pu4+ to Pu3+ starts only when americium reduction from Am4+ to Am3+ is completed. This result fills in an important gap in experimental data, thereby improving the thermodynamic description of nuclear fuels. At last, we suggest that the O/M ratio may evolve at room temperature, especially for high Am content, which is of main concern for the fabrication of Am-loaded MOX and their storage prior to irradiation.