Two pure mixed alkali/alkaline earth metal borates with three-dimensional framework, LiBaB9O15 and NaBaB9O15, have been synthesized by high-temperature solid state reaction, and characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of LiBaB9O15 and NaBaB9O15 in 1 mol L-1 HCl(aq), and of LiCl center dot H2O(s)/NaCl(s) in [1 mol L-1 HCl + H3BO3 + Ba(OH)(2)center dot 8H(2)O](aq) have been determined by microcalorimeter at 298.15 K, respectively. From these data and with the incorporation of the previously determined enthalpy of solution of H3BO3(S) in 1 mol L-1 HCl (aq), together with the use of the standard molar enthalpies of formation for Ba(OH)(2)center dot 8H(2)O(s), LiCl center dot H2O(s)/NaCl(s), H3BO3(s), HCl(aq) and H2O(I), the standard molar enthalpies of formation of -(6796.8 +/- 7.3) kJ mol(-1) for LiBaB9O15 and -(6829.9 +/- 73) kJ mol(-1) for NaBaB9O15 were obtained on the basis of the appropriate thermochemical cycles. (C) 2013 Elsevier B.V. All rights reserved.