Mixed crystals containing uranium and gadolinium for conversion into nuclear fuels by introducing NH.sub.3, CO.sub.2 and a uranyl salt solution containing dissolved gadolinium into a body of an aqueous solution of ammonium carbonate. As a result salts of gadolinium and uranium complexes precipitate and during precipitation there is a lowering of the pH from 9 down to 8.2. The precipitate is separated and converted to uranium and gadolinium oxides. Preparing the mixed crystals in this manner results in the gadolinium being finely and homogeneously distributed into the oxidic nuclear fuel.