Mixed oxide fuels are used in about 20 nuclear power plants around the world. After irradiation, americium that is generated co-exists within uranium dioxide. Due to the redox sensitive nature of UO₂, americium oxides other than AmO₂ are present in the fuel as a result of their interaction with the matrix. In this context, the aim of this study is to determine what americium species are present in heterogeneous and homogeneous MOX. The study of homogeneous MOX was approached by thermodynamics of solid solution of (Am,Pu,U)O₂. These solid solutions were modeled using the GEM-Selektor code supported by the literature data on such solid solutions. In the modeling study, the actinide oxides were set as ideal or as non-ideal solid solutions, as well as separate pure phases. Americium could very partially exist as Am(IV) <1% in the case of separate phases, but it yields its reduced forms, Am(III), in major fraction, in solid solutions and at temperature above 1000 K. The reduction of Am(lV) is driven by the reaction: 2AmO₂ + 4UO₂ → Am₂O₃ + 4UO_2.25. These results are confirmed by x-ray absorption spectroscopy (XAFS). The results provided by XAFS for non-irradiated as well as irradiated (fuel center and periphery) were published earlier for Pu and are completed by additional Am XAFS analysis in this work. The Am XAFS signals have been extracted using ATHENA code and the analyses were made as performed earlier. The analysis of the americium edge shows that the redox state of Am turns to trivalent in the solid solution and that the minor fraction of Am(IV) must be below 10%. This XAFS analysis confirms independently the results obtained so far by thermodynamic calculations.