The metastable solubilities and physicochemical properties (densities and refractive index) of the aqueous ternary system (Li2SO4 + MgSO4 + H2O) at 348.15 K were determined for the first time by the method of isothermal evaporation, and the metastable phase diagram and the diagrams of physicochemical properties versus composition were plotted. It was found that there is one invariant point of (Li2SO4 center dot H2O+MgSO4 center dot 4H(2)O), two univariant curves, and two crystallization regions corresponding to lithium sulfate monohydrate (Li2SO4 center dot H2O) and tetrahydrite (MgSO4 center dot 4H(2)O) in the metastable ternary system. The system belongs to a hydrate type I, and neither double salts nor solid solutions were formed. A comparison of the stable and metastable phase diagrams at this temperature shows that the metastable phenomenon of magnesium sulfate is obvious, and the crystallizing region of tetrahydrite is much smaller than that in the stable phase diagram. The solution density and refractive index of the metastable ternary system changes regularly with the mass fraction of lithium sulfate and reaches a maximum value at an invariant point. The densities and refractive index were correlated with empirical equations.