The enthalpies of formation were determined for Ca-4(PO4)(2)O, alpha-Ca-3(PO4)(2), and the hydroxyapatite compositions Ca-10(PO4)(6)(OH)(2) and Ca-9(HPO4)(PO4)(5)OH by the analysis of isothermal calorimetric results obtained at 37.4 degrees C. Both hydroxyapatite compositions were formed by reaction of Ca4PO4)(2)O with CaHPO4 or with CaHPO4 . 2H2O in water depending on the molar proportions of the reactant used. Ca-9(HPO4)PO4)(5)OH was also formed by direct hydrolysis of alpha-Ca-3(PO4)(2) in water. Enthalpy of formation values were calculated by a Hess’s Law analysis using several sets of equations. In summary, the enthalpy of formation values are : for Ca-4(PO4)(2)O, -476(2.4) kJ/mol, for alpha-Ca-3(PO4)(2), -409(6.5) kJ/mol, for Ca-9(HPO4)(PO4)(5)OH, -127(19.7) kJ/mol, and for Ca-10(PO4)(6)(OH)(2), -137(44.0) kJ/mol at 37.4 degrees C.