Solubility isotherms for the ternary system MgCl2-MgSO4-H2O and the quaternary reciprocal system Li+, Mg2+//Cl-, SO42--H2O were determined at 298.15 K by an isothermal dissolution method. In the ternary phase diagram, there are six solubility branches corresponding to the solid phases MgSO4 center dot nH(2)O((s)) (n = 7, 6, 5, 4, 1) and MgCl2 center dot 6H(2)O((s)). In the quaternary equilibrium phase diagram, there are 16 solubility co-saturated lines corresponding to the solid phases MgSO4 center dot nH(2)O((s)) (n = 7, 6, 5, 4, 1), MgCl2 center dot 6H(2)O((s)), Li2SO4 center dot H2O(s), LiCl center dot MgCl2 center dot 7H(2)O((s)), and LiCl center dot H2O(s). This report describes for the first time that the equilibrium solid phases MgSO4 center dot H2O(s) and MgSO4 center dot 4H(2)O((s)) have been found to exist in this quaternary system. However, the phase field of MgSO4 center dot H2O(s) overlaps with the phase fields of MgSO4 center dot 4H(2)O((s)) and MgSO4 center dot 5H(2)O((s)), which indicates that MgSO4 center dot 4H(2)O((s)) and MgSO4 center dot 5H(2)O((s)) are metastable phases; MgSO4 center dot H2O(s) is a relatively more stable phase in both the ternary and quaternary systems. A Pitzer-Simonson-Clegg thermodynamic model was used to simulate the properties of the sub-binary and subternary systems and to predict the solubility phase diagram of the quaternary system. The results of the modeling are in reasonable agreement with the experimental data.