The enthalpies of solution of NaRb[B4O5(OH)(4)]center dot 4H(2)O in approximately 1 mol dm(-3) aqueous hydrochloric acid and of RbCl in aqueous (hydrochloric acid + boric acid + sodium chloride) were determined. From these results and the enthalpy of solution of H3BP3 in approximately 1 mol dm(-3) HCl(aq) and of sodium chloride in aqueous (hydrochloric acid + boric acid), the standard molar enthalpy of formation of -(5128.02 +/- 1.94) kJ mol(-1) for NaRb[B4O5(OH)(4)]center dot 4H(2)O was obtained from the standard molar enthalpies of formation of NaCl(s), RbCl(s), H3BO3(s) and H2O(l). The standard molar entropy of formation of NaRb[B4O5(OH)(4)]center dot 4H(2)O was calculated from the Gibbs free energy of formation of NaRb[B4O5](OH)(4)]center dot 4H(2)O computed from a group contribution method. (c) 2006 Elsevier B.V. All rights reserved.