A method for preparing a powder comprising an intimate mixture of U3O8 particles and PuO2 particles and which may further comprise particles of ThO2 or NpO2. The method comprises: preparing, via oxalic precipitations, an aqueous suspension S1 of particles of uranium(IV) oxalate and an aqueous suspension S2 of particles of plutonium(IV) oxalate; mixing the aqueous suspension S1 with the aqueous suspension S2 to obtain an aqueous suspension S1+2; separating the aqueous suspension S1+2 into an aqueous phase and a solid phase comprising the particles of uranium(IV) oxalate and the particles of plutonium(IV) oxalate; and calcining the solid phase to convert (1) the particles of uranium(IV) oxalate to particles of triuranium octoxide and (2) the particles of plutonium(IV) oxalate to particles of plutonium(IV) dioxide, whereby the powder is obtained.