The enthalpy of dissolution of magnesium oxysulfate compound 2MgSO(4).MgO.3H(2)O in approximately 1 mol dm(-3) aqueous hydrochloric acid was determined. From this result and the enthalpies of dissolution of MgSO4.7H(2)O and MgO in the same solvent and the standard molar enthalpies of formation of MOW, MgSO4.7H(2)O(s) and H2O(l), the standard molar enthalpy of formation of 2MgSO(4).MgO.3H(2)O (-4132.07 +/- 0.57 kJ mol(-1)) was calculated. The standard molar entropy of formation of 2MgSO(4).MgO.3H(2)O was calculated from the thermodynamic relation with the standard molar Gibbs free energy of formation of 2MgSO(4).Mg.3H(2)O that was computed from a group contribution methods. (C) 2004 Elsevier B.V. All rights reserved.