Two pure mixed alkali-transition metal borates, Li6Zn3B4O12(s) and Na3ZnB5O10(s) have been synthesized and characterized by XRD, FT-IR, and chemical analysis. For Li6Zn3B4O12, the molar enthalpy of solution of Li6Zn3B4O12 in the mixture solvent of {2.00 mL of 1 mol.L-1 HCl + H3BO3)(aq)} was measured. The molar enthalpies of solution of ZnO(s) in 1 mol.L-1 HCl(aq), and LiBO2(s) in the mixture solvent of {2.00 mL of 1 mol.L-1 HCl(aq) + ZnO(aq)} were also measured. For Na3ZnB5O10, the molar enthalpy of solution of Na3ZnB5O10 in 2.00 mL of 1 mol.L-1 HCl(aq) was measured. The molar enthalpy of solution of NaBO2 center dot 4H(2)O(s) in the mixture solvent of {2.00 mL of 1 mol.L-1 HCl(aq) + H3BO3 + ZnO](aq)} was also measured. With the incorporation of the previously determined enthalpy of solution of H3BO3 (s) in 1 mol.L-1 HCl(aq), and ZnO(s) in {1 mol.L-1 HCl(aq) + H3BO3}, together with the use of the standard molar enthalpies of formation for LiBO2(s)/NaBO2 center dot 4H(2)O(s), ZnO(s), H3BO3(s) and H2O(l), the standard molar enthalpies of formation of -(5867.5 +/- 4.8) kJ.mol.L-1 for Li6Zn3B4O12 and -(4655.3 +/- 6.6) kJ.mol.L-1 for Na3ZnB5O10 at T = 298.15 K were obtained on the basis of two appropriate thermochemical cycles. (C) 2018 Elsevier Ltd.