Experimental studies on the metastable solubilities and the physicochemical properties (density, pH value, conductivity, and viscosity) of the aqueous reciprocal quaternary system (LiCl + MgCl2 + Li2SO4 + MgSO4 + H2O) at 323.15 K were determined with the isothermal evaporation method. According to the experimental results, the dry-salt phase diagram, water-phase diagram, and the physicochemical properties versus composition diagram were plotted. It was found that there are four invariant points, nine metastable solubility isotherm curves, and six metastable crystallization fields corresponding to lithium chloride monohydrate (LiCl center dot H2O), bischofite (MgCl2 center dot 6H(2)O), starkeyite (MgSO4 center dot 4H(2)O), hexahydrite (MgSO4 center dot 6H(2)O), lithium sulfate monohydrate (Li2SO4 center dot H2O), and the double salt lithium-carnallite (LiCl center dot MgCl2 center dot 7H(2)O). No solid solution was found. On the basis of the extended Harvie-Weare (HW) model and its temperature-dependent equation, the values of the Pitzer parameters beta((0)), beta((1)), beta((2)), and C-empty set for Li2SO4, MgCl2, and MgSO4, the mixed ion-interaction parameters theta(Li,Mg), Psi(Li,Mg,Cl), Psi(Li,Mg,SO4), Psi(Li,Cl,SO4) and Psi(Mg,Cl,SO4), and the Debye-Huckel parameter A degrees of the quaternary system at 323.15 K were obtained. The calculated solubility of the quaternary system (LiCl + MgCl2 + Li2SO4 + MgSO4 + H2O) at 323.15 K was done. A comparison between the calculated and the experimental results shows that the predicted solubility agrees well with the experimental data.