The enthalpies of solution of Rb2Ca[B4O5(OH)(4)](2) . 8H(2)O in approximately 1 mol dm(-3) aqueous hydrochloric acid and of RbCl in aqueous (hydrochloric acid + boric acid +calcium oxide) were determined. From these result and the enthalpies of solution of H3BO3 in approximately 1 mol dm(-3) HCl (aq), and of CaO in aqueous (hydrochloric acid + boric acid), the standard molar enthalpy of formation of -10, 308.06 . 6.57 kJ mol(-1) for Rb2Ca[B4O5(OH)(4)](2) . 8H(2)O was obtained from the standard molar enthalpy of formation of CaO (s), RbCl (s), H3BO3 (s) and H2O (1). The standard molar entropy of formation of Rb2Ca[B4O5(OH)(4)](2) . 8H(2)O was calculated from the thermodynamic relation with the standard molar Gibbs free energy of formation of Rb2Ca[B4O5(OH)(4)](2) . 8H(2)O computed from a group contribution method.