The crystalline form of a new tetraborate, NaCs[B 4O 5(OH) 4]·4H 2O was synthesized and characterized. The enthalpies of solution of NaCs[B 4O 5(OH) 4]·4H 2O in approximately 1 mol dm −3 HCl(aq) and of CsCl in aqueous (HCl + H 3BO 3 + NaCl) were determined. From these results and the enthalpy of solution of H 3BO 3 in approximately 1 mol dm −3 HCl(aq), the enthalpy of solution of NaCl(s) in aqueous (HCl + H 3BO 3) and the standard molar enthalpies of formation of NaCl(s), CsCl(s), H 3BO 3(s) and H 2O(l), the standard molar enthalpy of formation for NaCs[B 4O 5(OH) 4]·4H 2O as being −(5140.55 ± 1.94) kJ mol −1. The standard molar entropy of formation of NaCs[B 4O 5(OH) 4]·4H 2O was calculated from the thermodynamic relation with the standard molar Gibbs free energy of formation of NaCs[B 4O 5(OH) 4]·4H 2O computed from a group contribution method.