The enthalpies of solution of magnesium oxysulfate compounds MgSO4-5MgO-7H(2)O and MgSO4-5MgO-8H(2)O in approximately 1 mol dm(-3) aqueous hydrochloric acid were determined. From these results and the enthalpies of solution of MgSO4.7H(2)O and MgO in HCl(aq) respectively, the standard molar enthalpies of formation of -(6496.00 +/- 0.73) kJ mol(-1) for MgSO4-5MgO-7H(2)O and -(6800.17 +/- 0.55) kJ mol(-1) for MgSO4 . 5MgO-8H(2)O were obtained by using the standard molarenthalpies of formation of MgO(S), MgSO4 . 7H(2)O(s) and H2O(l). The standard molar entropies of formation of MgSO4- 5MgO . 7H(2)O and MgSO4 . 5MgO . 8H(2)O were calculated from the thermodynamic relation with the standard molar Gibbs free energy of formation Of MgSO4-5MgO-7H(2)O and MgSO4-5MgO-8H(2)O computed from a group contribution methods.