The crystalline hexamethylenetetramine pentaborate [(CH2)(6)N4H][B5O6(OH)(4)]center dot 0.5H(2)O has been prepared and identified by XRD, FT-IR, DTA-TG, elemental analysis and chemical analysis. The molar enthalpy of solution of [(CH2)(6)N4H][B5O6(OH)(4)]center dot 0.5H(2)O in (HCl center dot 54.554H(2)O) and of (CH2)(6)N-4 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 (CH2)(6)N-4(s), H3BO3(s), and H2O(l), the standard molar enthalpy of formation of -(4042.7 +/- 4.1) kJ mol(-1) of [(CH2)(6)N4H][B5O6(OH)(4)]center dot 0.5H(2)O was obtained. (C) 2005 Elsevier B.V. All rights reserved.