For nuclear fuel related applications, the oxygen stoichiometry of mixed oxides U1-yMyO2 +/- x, is an. essential property as it affects fuel properties and may endanger the safe: operation of nuclear reactors. A careful review of the open literature indicates that this parameter is difficult to assess properly and that the nature of the defects, i.e., Oxygen vacancies or U-v, in aliovalent cation-doped UO2 is still subject to controversy. To confirm the formation of U-v, we have investigated the room-temperature stable U1-yLayO2 +/- x phase using several experimental methods (e.g., XRD, XANES, and NMR) confirmed by theoretical calculations. This paper presents the experimental proof of U-V and its effect we identified in both electronic and local structure. We observe that Uv is formed in quasi-equimolar proportion as La in (y = 0.06, 0.11, and 0.22) solid solutions. The fluorite structure is maintained despite the cationic substitution, but the local structure is affected as variations of the interatomic distances are found. Therefore, we pro-Vide here the definitive proof that the substitution of U-IV with La-III is not accommodated by the creation of O vacancies as has often been assumed. The UO2 fluorite structure compensates the incorporation of an aliovalent cation by the formation of U-v in quasi-equimolar proportions.