Two pure barium borates, BaB2O4 center dot 4H(2)O and beta-BaB2O4, have been prepared and identified by X-ray diffraction, infrared spectral and thermogravimetric analysis, and chemical analysis. The molar enthalpies of solution of BaB2O4 center dot 4H(2)O and beta-BaB2O4 in 1 mol.dm(-3) HCl(aq) were measured to be -(7.63 +/- 0.10) kJ.mol(-1) and -(93.06 +/- 0.24) kJ.mol(-1), respectively. The molar enthalpy of solution of Ba(OH)(2)center dot 8H(2)O in (HCl + H3BO3)(aq) was determined to be -(55.42 +/- 0.36) kJ.mol(-1). With the incorporation of the enthalpy of solution of H3BO3 in 1 mol.dm(-3) HCl(aq) and the standard molar enthalpies of formation of Ba(OH)(2)center dot 8H(2)O(s), H3BO3(s), and H2O(l), the standard molar enthalpies of formation of BaB2O4 center dot 4H(2)O and beta-BaB2O4 were found to be -(3249.1 +/- 1.7) kJ.mol(-1) and -(2020.3 +/- 1.7) kJ.mol(-1), respectively.