The enthalpies of solution of Cs-2[B4O5(OH)(4)] center dot 3H(2)O in HCl(aq), and of CsCl in (HCl + H3BO3)(aq) were determined. With the incorporation of the previously determined enthalpy of solution of H3BO3 in HCl(aq) and the standard molar enthalpies of formation of CsCl(s), H3BO3(s), HCl(aq), and H2O(l), the standard molar enthalpy of formation of Cs-2[B4O5(OH)(4)] 3H2O of -(4859.23 +/- 0.64) kJ center dot mol(-1) was obtained. Thermodynamic properties of this compound were also calculated by a group contribution method. (c) 2005 Elsevier Ltd. All rights reserved.