Several compositions of Th(2-x/2)An(x/2)(IV)(PO4)(2)(HPO4)center dot H2O (An = U, Np, Pu) were prepared through hydrothermal precipitation from a mixture of nitric solutions containing cations and concentrated phosphoric acid. All the samples were fully characterized by X-ray diffraction, UV-vis, and infrared spectroscopies to check for the existence of thorium-actinide(IV) phosphate hydrogenphosphate hydrates solid solutions. Such compounds were obtained as single phases, up to x = 4 for uranium, x = 2 for neptunium, and x < 4 for plutonium, the cations being fully maintained in the tetravalent oxidation state. In a second step, the samples obtained after heating crystallized precursors at high temperature (1100 degrees C) were characterized. Single-phase thorium-actinide(IV) phosphate-diphosphate solid solutions were obtained up to x = 0.8 for Np(IV) and x = 1.6 for Pu(IV). For higher substitution rates, polyphase systems composed by beta-TAnPD, An(2)O(PO4)(2), and/or alpha-AnP(2)O(7) were formed. Finally, this hydrothermal route of preparation was applied successfully to the synthesis of an original phosphate-based compound incorporating simultaneously tetravalent uranium, neptunium and plutonium.