Trimagnesium phosphate Mg-3(PO4)(2) (triMg) and beta tricalcium phosphate Ca-3(PO4)(2) (triCa) have been prepared and characterized by X-ray diffraction, IR spectroscopy and chemical analysis. Using an isoperibol calorimeter the heat of solution of these products in a 9% weight nitric acid solution were determined. They are (-232.6 +/- 2.7) kJ mol(-1), (-130.3 +/- 1.2) kJ mol(-1), respectively. Combining these results with those found by dissolving other products and entities involved in thermochemical cycles, allows to determine the standard enthalpies of formation of these phosphates. These quantities are -3706 kJ mol(-1) for triMg and -4056 kJ mol(-1) for triCa. These values together with those obtained previously for triSr and triBa were discussed taking into account the coordination number and the metal-oxygen bond energy.