Calcium-strontium fluorapatites solid solutions, having the general formula Ca10-xSrx(PO4)(6)F-2, were prepared and characterized by X-ray diffraction, infrared spectroscopy and chemical analysis. Using an isoperibol calorimeter, enthalpies of solution of these products in 9% nitric acid solution were measured. The standard enthalpy of formation of these apatites and the enthalpy of mixing of end-member products were deduced. The variation of all these quantities vs. composition showed a clear change at Sr/(Sr + Ca) close to 0.6. This result has been related to the preferential occupation of the sixfold metal position by strontium in the lattice.