A new hydrated double borate, K2Ba[B4O5(OH)(4)](2) center dot 8H(2)O, has been synthesized and characterized by chemical analysis, elemental analysis, X-ray diffraction, infrared spectrum, and thermogravi metric analysis. The molar enthalpies of solution of K2Ba[B4O5(OH)(4)]2 center dot 8H(2)O in 1 mol.dm(-3) HCl(aq) and of KCl(s) in [Ba(OH)(2)center dot 8H(2)O(aq) + H3BO3(aq) + HCl(aq)] were measured to be (90.74 +/- 0.27) kJ . mol(-1) and (17.37 +/- 0.31) kJ.mol(-1), respectively. With the incorporation of the previously determined enthalpy of solution of Ba(OH)(2)center dot 8H(2)O(S) in [HCl + H3BO3](aq) and that of H3BO3(S) in HCl(aq), together with the standard molar enthalpies of formation of HCl(aq), H2O(1), Ba(OH)(2)center dot 8H(2)O(S), H3BO3(s), and KCl(s), the standard molar enthalpy of formation of K2Ba[B4O5(OH)(4)](2)center dot 8H(2)O was found to be -(10295.2 +/- 6.5) kJ.mol(-1). The standard molar free energy of formation was estimated to be -9217.53 kJ.mol(-1). The standard molar entropy was estimated to be 858.1 J.mol(-1).K-1.