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U 3 O 8 is considered to be the most stable phase for uranium oxide. Its structural properties must be accurately understood to foresee and manage aspects such as its leaching behavior when spent nuclear fuel is stored in an oxidative environment. Moreover, as fuel irradiation causes the formation of fission products and activation products such as plutonium and minor actinides, it is probable that U 3 O 8 will be mixed with other chemical elements under real conditions of oxidation. The storage issue can be extended to americium transmutation, where the irradiated compounds are mixed oxides composed of uranium and americium. This study thus focused on determining the structural properties of a solid solution containing uranium and trivalent americium (U/Am ratio = 90/10) and synthesized so as to obtain conventional U 3 O 8 oxide. This paper presents the possibility of combining trivalent americium with uranium in a U 3 O 8 mixed oxide for the first time, despite the high valence and atomic ratio differences, and proposes novel structural arrangements. X-ray diffraction measurements reveal americium substitution in U 3 O 8 uranium cationic sites, leading to phase transformation into a U 3 O 8 high-temperature structure and general lattice swelling. X-ray absorption near-edge spectroscopy and extended X-ray absorption fine structure experiments highlight an excess of U +VI organized in uranyl units as the main consequence of accommodation.