Calcium-cadmium hydroxyapatite solid solutions with the general formula Ca10-xCdx(PO4)6(OH)2 were prepared by a double decomposition method. The samples were characterized by X-ray diffraction, infrared spectroscopy and elemental analysis. Using an isoperibol calorimeter, enthalpies of solution of these products in 10% nitric acid solution were measured. The combination of these results with those of the limiting products x = 0 and x = 10 allows the determination of the enthalpies of mixing. Reaction schemes are proposed in order to determine the standard enthalpy of formation of cadmium and calcium hydroxyapatites.