Two transition metal borates with microporous structures for Na2Co2B12O21 and Na2Cu2B12O21 have been synthesized and characterized by XRD, FT-IR, and elemental analysis. The molar enthalpies of solution of Na2Co2B12O21(s) and Na2Cu2B12O21(s) in 1 mol dm(-3) HCl(aq) were measured by microcalorimeter at T = 298.15 K, respectively. The molar enthalpies of solution of CoCl2 center dot 6H(2)O(s) and CuCl2 center dot 2H(2)O(s) in the mixture solvent of 2.00 cm(3) of 1 mol dm(-3) HCl(aq) in which 5.30 mg of H3BO3 and 0.84 mg of NaCl being added were also measured, respectively. With the incorporation of the previously determined enthalpy of solution of H3BO3(s) in 1 mol dm(-3) HCl(aq), together with the standard molar enthalpies of formation for CoCl2 center dot 6H(2)O(s)/CuCl2 center dot 2H(2)O(s), NaCl(s), H3BO3(s), HCl(aq) and H2O(l), the standard molar enthalpies of formation of - (9072.4 +/- 9.7) kJ mol(-1) for Na2Co2B12O21(s) and - (8887.5 +/- 9.7) kJ mol(-1) for Na2Cu2B12O21(s) at T = 298.15 K were obtained on the basis of the appropriate thermochemical cycles. It is obvious that Na2Co2B12O21(s) is more thermal stable than that of Na2Cu2B12O21(s). (C) 2016 Elsevier Ltd.