Ammonium-iron(II) phosphate monohydrate was synthesized via solid-state reaction at low temperature and characterized by X-ray diffraction, infrared spectroscopy, chemical analysis and scanning electron microscope. Thermochernical study was performed by an isoperibol solution calorimeter at 298.15 K. The results show that the obtained product is NH4FePO4 center dot H2O with plate shape of nanometer measure. According to Hess's law, a new thermochemical cycle was designed, and the standard enthalpy of formation of the titled complex at 298.15 K is Delta fH(m)(circle minus) [NH4FePO4 center dot H2O(s), 298.15 K] = -1774.92 +/- 0.39 kJ mol(-1). (C) 2007 Published by Elsevier Ltd.