A pure hydrated calcium double borate, (NH4)(2)Ca[B4O5(OH)(4)](2), 8H(2)O, has been synthesized and identified by chemical analysis, elemental analysis, X-ray diffraction, infrared spectra, and thermogravimetric analysis. The molar enthalpy of solution of (NH4)(2)Ca[B4O5(OH)(4)](2), 8H(2)O in 1 mol(.)dm(-3) HCl(aq) and of NH4Cl(s) in (HCl + H3BO3 + CaO)(aq) were measured to be (104.50 +/- 0.34) kJ, mol(-1) and (17.71 +/- 0.30) kJ, mol(-1), respectively. From a combination of these results with measured enthalpies of solution of H3BO3(s) in 1 mol(.)dm(-3) HCl(aq) and of CaO( s) in ( HCl + H3BO3) aqueous solution together with the standard molar enthalpies of formation of HCl(aq), NH4Cl(s), CaO(s), H3BO3(s), and H2O(l), the standard molar enthalpy of formation of (NH4) 2Ca[B4O5( OH)(4)](2), 8H(2)O was found to be-(10062.3 +/- 6.5) kJ, mol(-1). Thermodynamic properties of the compound were also estimated by a group contribution method. The standard molar free energy of formation (Delta(f)G(m)degrees) was estimated to be -8802.38 kJ(.)mol(-1). From these values, the standard molar entropy of formation (Delta S-f(m)degrees) was estimated to be -4225.8 J(.)mol(-1.)K(-1). The standard molar entropy (S-m degrees) was estimated to be 809.3 J(.)mol(-1), K-1.