A pure calcium borate Ca-2[B2O4(OH)(2)]center dot 0.5H(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 Ca-2 [B2O4(OH)(2)]center dot 0.5H(2)O in HCl center dot 54.582H(2)O was determined. From a combination of this result with measured enthalpies of solution of H3BO3 in HCl center dot 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(l), the standard molar enthalpy of formation of -(3172.5 +/- 2.5) kJ mol(-1) of Ca-2[B2O4(OH)(2)]center dot 0.5H(2)O was obtained. (C) 2005 Elsevier B.V. All rights reserved.