The heats of solution of beta-tricalcium phosphate Ca-3(PO4)(2) (beta-TCP) and hydroxyapatite Ca-10(PO4)(6)(OH)(2) (HAp) in phosphoric acid solutions of different concentration, were measured using an isoperibolic calorimeter. These values are -125.5 +/- 2.1, -122.0 +/- 0.5, -116.4 +/- 0.5 kJ mol(-1) for beta-TCP and -354.6 +/- 0.5, -342.1 +/- 0.5, -337.8 +/- 1.4 kJ mol(-1) for HAp in (H3PO4; 13.55 H2O), (H3PO4; 6.90 H2O) and (H3PO4; 4.09 H2O) respectively. Combination of these results with the enthalpies of solution of monocalcium phosphate monohydrate Ca(H2PO4)(2).H2O and H2O in the same solvents allows us to propose a mechanism for the dissolution of these phosphates in phosphoric acid.