Two pure zinc borates with microporous structure 3ZnO = 3B2O3 = 3.5H2O and 6ZnO = 5B2O3 = 3H2O have been synthesized and characterized by XRD, FT-IR, TG techniques and chemical analysis. The molar enthalpies of solution of 3ZnO = 3B2O3 = 3.5H2O(s) and 6ZnO = 5B2O3 = 3H2O(s) in 1 mol = dm = 3 HCl(aq) were measured by microcalorimeter at T = 298.15 K, respectively. The molar enthalpies of solution of ZnO(s) in the mixture solvent of 2.00 cm3 of 1 mol = dm = 3 HCl(aq) in which 5.30 mg of H3BO3 were added were also measured. With the incorporation of the previously determined enthalpy of solution of H3BO3(s) in 1 mol = dm = 3 HCl(aq), together with the use of the standard molar enthalpies of formation for ZnO(s), H3BO3(s), and H2O(l), the standard molar enthalpies of formation of = (6115.3 = 5.0) kJ = mol = 1 for 3ZnO = 3B2O3 = 3.5H2O and = (9606.6 = 8.5) kJ = mol = 1 for 6ZnO = 5B2O3 = 3H2O at T = 298.15 K were obtained on the basis of the appropriate thermochemical cycles. (C) 2014 Elsevier Ltd. All rights reserved.