The enthalpies of solution of Rb 2 Ca[B 4 O 5 (OH) 4 ] 2 .8H 2 O in approximately 1moldm - 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 H 3 BO 3 in approximately 1moldm - 3 HCl (aq), and of CaO in aqueous (hydrochloric acid+boric acid), the standard molar enthalpy of formation of -10,308.06+/-6.57 kJmol - 1 for Rb 2 Ca[B 4 O 5 (OH) 4 ] 2 .8H 2 O was obtained from the standard molar enthalpy of formation of CaO (s), RbCl (s), H 3 BO 3 (s) and H 2 O (l). The standard molar entropy of formation of Rb 2 Ca[B 4 O 5 (OH) 4 ] 2 .8H 2 O was calculated from the thermodynamic relation with the standard molar Gibbs free energy of formation of Rb 2 Ca[B 4 O 5 (OH) 4 ] 2 .8H 2 O computed from a group contribution method.