In order to reduce the nuclear waste inventory and radiotoxicity, U1-xAmxO2 +/-delta materials are promising fuels for heterogeneous transmutation. In this context, they are generally fabricated from UO2+delta and AmO2-delta dioxide powders. In the subsequent solid solution, americium is assumed to be trivalent whereas uranium exhibits a mixed-valence (+IV/+V) state. However, no formation mechanisms were ever evidenced and, more particularly, it was not possible to know whether the reduction of Am(IV) to Am(III) occurs before the solid-solution formation, or only once it is established. In this study, we used high-temperature X-ray diffraction on a UO2 +/-delta/AmO2-delta (15 mol %) mixture to observe in situ the formation of the U1-xAmxO2 +/-delta solid solution. We show that UO2+delta is, at relatively low temperature (<700 K), oxidized to U4O9-delta, which is likely to be caused by oxygen release from the simultaneous AmO2-delta reduction to cubic Am2O3 +/-delta. Cubic Am2O3+delta then transforms to hexagonal Am2O3 at 1300 K Thus, the initial Am(IV) is fully reduced to Am(III) before the solid solution starts forming at 1740 K The UO2 fluorite phase vanishes after 4 h at 1970 K, indicating that the formation of the solid solution is completed, which proves that this solid solution is formed after the complete reduction of Am(IV) to Am(III).