Ammonium/transition metal phosphate monohydrate NH4MPO4 center dot H2O (M = Mn2+, Co2+, Ni2+, Cu2+) compounds were synthesized by solid-state reaction at low temperature and characterized by X-ray diffraction (XRD), FT-IR, and chemical analysis. Based on Hess's law, the thermochemical cycles were designed for measuring the dissolution enthalpies of reactants and products using a solution-reaction isoperibol calorimeter at 298.15 K. From the dissolution enthalpies, the molar enthalpies of the five reactions designed were calculated individually. With these data and other auxiliary theromodynamic data, the standard molar enthalpies of formation of the title compounds were concluded as: Delta H-f(m)Theta[NH4MnPO4 center dot H2O] = (-1925.01 0.25) kJ.mol(-1), Delta H-f(m)Theta[NH4CoPO4 center dot H2O] = (-1754.91 +/- 0.43) kJ.mol(-1), Delta H-f(m)Theta[NH4NiPO4 center dot H2O] (-1755.48 +/- 0.39) kJ.mol(-1), and Delta H-f(m)Theta[NH4CuPO4 center dot H2O] = (-1636.41 +/- 0.36) kJ.mol(-1).