A new magnesium borate MgO.3B(2)O(3).3.5H(2)O has been synthesized by the method of phase transformation of double salt and characterized by XRD, IR and Raman spectroscopy as well as by TG. The structural formula of this compound was Mg[B6O9(OH)(2)].2.5H(2)O. The enthalpy of solution of MgO.3B(2)O(3).3.5H(2)O in approximately 1 mol dm(-3) HCl was determined. With the incorporation of the standard molar enthalpies of formation of MgO(s), H3BO3(s), and H2O(1), the standard molar enthalpy of formation of -(5595.02 +/- 4.85) kJmol(-1) of MgO.3B(2)O(3)3.5H(2)O was obtained. Thermodynamic properties of this compound was also calculated by group contribution method. (C) 2003 Elsevier Science B.V. All rights reserved.