Isopiestic determinations of the osmotic coefficient of MgSO4(aq) against NaCl(aq) and KCl(aq) reference standards were performed with long equilibration times ranging to 98 days. A value of the 298.15 K solubility of epsomite, MgSO4. 7H(2)O(cr), was also determined. The new measurements were combined with previous thermodynamic measurements to determine the thermodynamic properties of the MgSO4 + H2O system from the ice-freezing line to 440 K. The thermodynamic measurements were fitted with two different models, an ion-interaction model and a chemical-equilibrium model. The ability of the two models to represent this system was examined.