The crystalline triimidazolium nonaborate [C3H5N2](3)[B9O12(OH)(6)] has been prepared and identified by XRD, TG, elemental analysis and chemical analysis. The molar enthalpy of solution of [C3H5N2](3)[B9O12(OH)(6)] in 0.9996 mol dm(-3) HCl(aq) and of imidazole in (HCl + H3BO3) (aq) were determined. With the incorporation of the enthalpies of solution of H3BO3 in HCl(aq), and the standard molar enthalpies of formation Of C3H4N2(s), H3BO3(s), and H2O(l), the standard molar enthalpy of formation of -7116 +/- 70 mol(-1) of [C3H5N2](3) [B9O12(OH)(6)] was obtained. (c) 2005 Elsevier B.V. All rights reserved.