The enthalpies of solution of Rb2B4O7.5.6H(2)O in approximately 1 mol dm(-3) aqueous hydrochloric acid and of RbCl in aqueous (hydrochloric acid+boric acid) were determined. From these results and the enthalpy of solution of H3BO3 in approximately 1 mol dm(-3) HCl(aq), the value of the molar enthalpy of formation of -5023.36 +/- 3.26 kJ mol(-1) for Rb2B4O7.5.6H(2)O was obtained by using the values for the standard molar enthalpies of formation of RbCl(s), H3BO3(s), and H2O(l). The standard molar entropy of formation of Rb2B4O7.5.6H(2)O was calculated from the thermodynamic relation of the standard molar enthalpy of formation with the standard molar Gibbs free energy of formation of Rb2B4O7.5.6H(2)O computed from a group contribution method. (C) 2003 Elsevier Science B.V. All rights reserved.