Microcrystals of (NH4)(2)Ce(PO4)(2)center dot H2O were hydrothermally obtained from a CeO2-CO(NH2)(2)-H3PO4-H2O system (T = 180 degrees C). The structure [orthorhombic, Imma, a = 6.8940(9), b = 6.8860(9), c = 17.723(2) angstrom] consists of CeO8 distorted dodecahedra and PO4 tetrahedra joined together to form a three-dimensional framework with small four-sided and larger six-sided tunnels in quasi-equivalent directions. Ammonium ions and water molecules are situated inside the larger tunnels. Also, the thermal decomposition is discussed.