Two pure mixed alkali metal borates with Non-Linear Optical (NLO) properties, Li6Rb5B11O22 and Li4Cs3B7O14, have been synthesized by a high-temperature solid state reaction, and characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of Li6Rb5B11O22 and Li4Cs3B7O14 in 1 mol . dm(-3) HCl (aq), and of LiCl center dot H2O(s) in (1 mol . dm(-3) HCl + H3BO3 + RbCl/CsCl) (aq) have been determined by microcalorimeter at T = 298.15 K, respectively. From these data and 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 LiCl center dot H2O(s), RbCl(s)/CsCl(s), H3BO3(s), HCl(aq) and H2O(l), the standard molar enthalpies of formation of -(11173.1 +/- 9.5) kJ . mol(-1) for Li6Rb5B11O22 and -(7145.1 +/- 5.9) kJ . mol(-1) for Li4Cs3B7O14 were obtained on the basis of the appropriate thermochemical cycles. (c) 2013 Elsevier Ltd. All rights reserved.