Several compositions of thorium–uranium (IV) phosphate–hydrogenphosphate hydrate (Ux/2Th2−x/2(PO4)2(HPO4) · H2O, TUPHPH) were prepared starting from actinides chloride solutions and concentrated phosphoric acid. The experimental synthesis parameters were optimized in order to get the quantitative precipitation of the cations and a good crystallization state. The extensive characterization of the solids demonstrated the existence of a complete solid solution between Th and U end-members and evidenced the good homogeneity of the powders. Their behaviors during heating treatment were then checked and confirm the formation of anhydrous thorium–uranium (IV) phosphate–hydrogenphosphate (TUPHP) and α-UxTh4−x(PO4)4(P2O7) (α-TUPD) acting as intermediates. Finally, the low-temperature crystallized precursors were used in original sintering processes in order to improve the efficiency of the former cold-pressing sintering procedure.