In U1-xAmxO2 +/-delta compounds with low americium content (x <= 20 atom %) and oxygen-to-metal (O/M) ratios close to 2.0, AmIII+ cations are charge-balanced by an equivalent amount of UV+ cations while the fluorite structure of pure UIV+O2 is maintained. Up to now, it is unknown whether this observation also holds for higher americium contents. In this study, we combined X-ray diffraction with Raman and X-ray absorption spectroscopies to investigate a U0.5Am0.5O2 +/-delta compound. Our results indicate that americium is again only present as AmIII+, while UV+ remains below the amount required for charge balance. Unlike lower americium contents, this leads to an overall oxygen hypostoichiometry with an average O/M ratio of 1.92(2). The cationic sublattice is only slightly affected by the coexistence of large amounts of reduced (AmIII+) and oxidized (UV+) cations, whereas significant deviations from the fluorite structure are evidenced by both extended X-ray absorption fine structure and Raman spectroscopies in the oxygen sublattice, with the observation of both vacancies and interstitials, the latter being apparently consistent with the insertion of U6O12 cuboctahedral-type clusters (as observed in the U4O9 or U4O9 or U3O7 phases). These results thus highlight the specificities of uranium-americium mixed oxides, which behave more like trivalent lanthanide-doped UO2 than U1-xPuxO2 +/-delta MOX fuels.