Th(2)(PO(4))(2)(HPO(4)).H(2)O was synthesized under wet hydrothermal conditions starting from a mixture of H(3)PO(3) and Th(NO(3))(4).5H(2)O. The crystal structure was solved by powder X-ray diffraction data. The unit cell parameters are a = 6.7023(8) Angstroms, b = 7.0150(8) Angstroms, c = 11.184(1) Angstroms, beta = 107.242(4) degrees, space group P2(1), and Z = 2. The structure consists of layers of both thorium atoms and PO(4) groups, alternating with a layer formed by HPO(4) entities and water molecules. By thermal treatment, this compound turns into Th(4)(PO(4))(4)P(2)O(7), a ceramic already described in the field of the immobilization of tetravalent actinides.