Th-2(PO4)(2)(HPO4)-H2O was synthesized under wet hydrothermal conditions starting from a mixture of H3PO3 and Th(NO3)(4)• 5H(2)O. The crystal structure was solved by powder X-ray diffraction data. The unit cell parameters are a = 6.7023(8) &ANGS;, b = 7.015(8) &ANGS;, c = 11.184(1) &ANGS;, β = 107.242(4)°, space group P2(1), and Z = 2. The structure consists of layers of both thorium atoms and PO4 groups, alternating with a layer formed by HPO4 entities and water molecules. By thermal treatment, this compound turns into Th-4(PO4)(4)P2O7, a ceramic already described in the field of the immobilization of tetravalent actinides.