Two mixed alkali-alkaline earth metal borates of NaSrBO3 and KSr4B3O9 have been synthesized by high-temperature solid state reaction, which were further characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of NaSrBO3(s) and KSr4B3O9(s) in 2.00 cm(3) of 1 mol center dot dm (3) HCl(aq), at T = 298.15 K were measured to be -(206.84 +/- 0.43) kJ center dot mol (1) and -(494.59 +/- 0.53) kJ center dot mol (1), respectively. The molar enthalpy of solution of NaCl(s) in 2.00 cm(3) of {1 mol center dot dm (3) HCl + H3BO3 + Sr(OH)(2) center dot 8H(2)O}(aq) mixed solvent at T = 298.15 K was measured to be (5.17 +/- 0.02) kJ center dot mol (1). From these data and with the incorporation of the previously determined enthalpies of solution of H3BO3(s) in HCl(aq) of Sr(OH)(2) center dot 8H(2)O(s) in (HCl + H3BO3)(aq), and of KCl(s) in {HCl + H3BO3 + Sr(OH)(2) center dot 8H(2)O}(aq), together with the use of the molar enthalpies of formation for NaCl(s)/KCl(s), Sr(OH)(2) center dot 8H(2)O(s), H3BO3(s), HCl(aq) and H2O(l), the standard molar enthalpies of formation of NaSrBO3(s) and KSr4B3O9 were calculated to be -(1653.1 +/- 1.4) kJ center dot mol (1) and -(5071.1 +/- 3.4) kJ center dot mol (1) on the basis of the designed thermochemical cycles, respectively. (C) 2013 Elsevier Ltd. All rights reserved.