A thermodynamic electrolyte model has been used to investigate the system MgSO4 + H2O at temperatures close to the eutectic from (0 to -10) degrees C. The model was based on the Pitzer aqueous electrolyte activity coefficient model using the interaction parameters proposed by Marion and Farren (Marion, G.; Farren, R. E. Mineral solubilities in the Na-K-Mg-Ca-Cl-SO4-H2O system: A re-evaluation of the sulfate chemistry in the Spencer-Moller-Weare model. Geochim. Cosmochim. Acta 1999,63,1305-1318) as well as improved functions that have been derived in this work. Because of its improved data fitting, the model describes the System MgSO4 + H2O accurately from (-5 to 50) degrees C. By making use of solubility data obtained from the literature as well as from additional laboratory experiments, we investigated the magnesium sulfate duodecahydrate solid phase (MgSO(4)(.)12H(2)O). The two characteristic points, which limit the MgSO(4)(.)12H(2)O(s) region of existence, are the eutectic point between MgSO(4)(.)12H(2)O(s) and ice and the phasetransition point (peritectic) between MgSO(4)(.)12H(2)O(s) and MgSO4-7H20(S) (epsomite, bitter salt). Both were determined from the solubility experiments and the model. The solubility products (K-sp) of MgSO(4)(.)12H(2)O(s) and MgSO(4)(.)7H(2)O(s) were calculated as functions of temperature from solubility and phase stability data. These functions improve K-sp functions published by Marion and Farren (Marion, G.; Farren, R. E. Mineral solubilities in the Na-K-Mg-Ca-Cl-SO4-H2O system: A re-evaluation of the sulfate chemistry in the Spencer-Moller-Weare model. Geochim. Cosmochim. Acta 1999, 63, 1305-1318). It was experimentally determined that the eutectic occurs at 17.4 mass % MgSO4 and -3.7 degrees C, and the presented model predicts it at 17.3 mass % MgSO4 and -3.7 degrees C.