A pure calcium borate Ca-2[B-4 O-7(OH)(2)] 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 Ca-2[B4O7(OH)(2)] in HCl.54.549H(2)O was determined. From a combination of this result with measured enthalpies of solution of H3BO3 in HCl.54.517H(2)O and of CaO in (HCl+H3BO3) solution, together with the standard molar enthalpies of formation of CaO(s), H3BO3(s), and H2O(l), the standard molar enthalpy of formation of -(4398.9 +/- 3.8) kJ mol(-1) for Ca-2[B4O7(OH)(2)] was obtained. (c) 2005 Elsevier B.V. All rights reserved.