A pure calcium borate 2CaO(.)B(2)O(3)(.)H(2)O has been synthesized under hydrothermal condition and characterized by XRD, FT-IR and TG as well as by chemical analysis. The molar enthalpy of solution of 2CaO.(B2O3H2O)-H-. in HCI-54.579H20 was determined. From a combination of this result with measured enthalpies of solution of H3BO3 in HCI(.)54(.)561H(2)O and of CaO in HCl+H3BO3 solution, together with the standard molar enthalpies of formation of CaO(s), H3BO3(s), and H2O(1), the standard molar enthalpy of formation of -(3041.8 +/- 2.5) kJ mol(-1) of 2CaO(.)B(2)O(3)(.)H(2)O was obtained. (C) 2004 Elsevier B.V. All rights reserved.