Partitioning and transmutation of Minor Actinides (MA) are currently studied to reduce radiotoxicity and heat generation in nuclear wastes. In the frame of minor actinides recycling in sodium fast reactors, (U,Am)O2 mixed oxides are promising transmutation targets. To assess the thermodynamic properties of the U-Am-O system, it is essential to have a thorough knowledge of the binary phase diagrams, which is difficult due to the lack of thermodynamic data on the Am-O system1. Nevertheless, a thermodynamic modelling of the Am-O system has been recently proposed by Gotcu-Freiss et al.2 AmO2-x (fcc), AmO1.62 (bcc) and Am2O3 (hcp) phases are reported for an Oxygen/Americium (O/M) ratio ranging from 2 to 1.5. Here, we investigated the Am-O system using in situ high temperature X-ray diffraction. First, we analyzed the evolution of the lattice parameter of the AmO2-x (fcc) phase under air in temperature up to 1873 K. Below 1200K, the lattice parameter change is associated only to the temperature effect and the thermal expansion coefficient was calculated. Above this temperature, we observed a steeper increase of the lattice parameter, which is due to a reduction of the phase, i.e. a lower O/M, as predicted by our thermodynamic calculations based on the Gotcu-Freiss model. Second, we explored the AmO2-x region (1.6