Alkaline earth chloroapatites, M-10(PO4)(6)Cl-2 (M = Ba, Sr, Ca) were prepared by solid state reaction route and characterized by XRD. The lattice constants of Ba-10(PO4)(6)Cl-2. Sr-10(PO4)(6)Cl-2. Ca-10(PO4)(6)Cl-2 were determined to be a = 10.27(1) angstrom, c = 7.66(1) angstrom; a = 9.87(2) angstrom, c = 7.18(1) angstrom and a = 9.52(1) angstrom, c = 6.85(1) angstrom respectively. The enthalpy increments were measured by inverse drop calorimetric method in the temperature range 523-1423 K using a high temperature calorimeter. Thermodynamic functions such as heat capacity, entropy and Gibbs energy functions in the temperature range 298-1500 K. were computed from the measured enthalpy increments. (C) 2011 Elsevier B.V. All rights reserved.