A process for synthesizing a mixed oxide of an actinide An selected from the group consisting of uranium and neptunium, and of at least one metal able to form a doubly, triply or quadruply charged metal cation X1, the metal differing from An, which process comprises:
- synthesizing a mixed peroxide or hydroxo-peroxide of an actinyl of formula AnO2q+ where q equals 1 or 2, and of at least the cation X1, the synthesis comprising a reaction in a solvent of a salt of the cation X1 with a compound C2 selected from the group consisting of mixed peroxides and hydroxo-peroxides of the actinyl and of at least one singly charged cation X2, whereby compound C2 is converted to compound C1 by a replacement of the cation X2 by the cation X1;
- and
- calcining the mixed peroxide or hydroxo-peroxide thus synthesized.