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Experimental studies on the metastable solubilities and physicochemical properties (densities and refractive index) of the aqueous ternary system (Li2SO4 + MgSO4 + H2O) at 273.15 K were determined by the method of isothermal evaporation. According to the experimental results, 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·H2O + MgSO4·7H2O), two univariant curves, and two crystallization regions corresponding to lithium sulfate monohydrate (Li2SO4·H2O) and epsomite (MgSO4·7H2O) in the metastable ternary system. The system belongs to a hydrate type I, and neither double salts nor solid solutions were formed. The solution density and refractive index of the metastable ternary system changes regularly with the mass fraction of magnesium sulfate and all reach the maximum value at the invariant point. The calculated values of density and refractive index with the empirical equations are in good agreement with the experimental values.