Tristrontium phosphate Sr-3(PO4)(2) (triSr) and strontium chlorapatite Sr-3(PO4)(6)Cl-2 (ClapSr) 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 (-110.1 +/- 1.5) and (-283.6 +/- 2.4) kJ mol(-1) for triSr and ClapSr, respectively. Combining these results with others found by dissolving the products and entities involved in thermochemical cycles, allows to determine the standard heat of formation of these Sr-phosphates. These quantities are : Delta(f)H degrees(triSr, 298 K) = -4110 kJ mol(-1) and Delta(f)H degrees(ClapSr, 298 K) = -13233 kJ mol(-1). The results are discussed on the basis of the differences in bond enthalpies.