The enthalpy of solution of ulexite NaCa[B5O6(OH)(6)]5 . H2O in approximate to 1 mol dm(-3) aqueous hydrochloric acid has been measured. From this result and the enthalpies of solution of H(3)BO(0)3 in HCl(aq), of CaO in aqueous (hydrochloric acid + boric acid), and of NaCl in aqueous (hydrochloric acid + boric acid + calcium oxide), the standard molar enthalpy of formation of ulexite of -6762.24 +/- 4.22 kJ mol(-1) has been obtained. The standard molar entropy of formation of ulexite was calculated from the thermodynamic relation with the standard molar Gibbs free energy of formation of ulexite computed from its solubility in water.