Considering that phosphate matrices could be potential candidates for the immobilization of actinides or for the final disposal of the excess plutonium from dismantled nuclear weapons, the chemistry of thorium phosphates has been re-examined. In the ThO/sub 2/-P/sub 2/O/sub 5/ system, the thorium phosphate-diphosphate Th/sub 4/(PO/sub 4/)/sub 4/P/sub 2/O/sub 7/ (TPD) can be synthesized by wet and dry chemical processes. The substitution of thorium by other tetravalent actinides like uranium or plutonium can be obtained for 0