The metastable solubilities and the physicochemical properties (density, viscosity, refractive index, conductivity, and pH) of the aqueous quaternary system (Li2SO4 + K2SO4 + MgSO4 + H2O) at 288.15 K were determined with the isothermal evaporation method. According to the experimental data, the dry-salt phase diagram, water-phase diagram, and the physicochemical properties vs composition diagram were plotted. It was found that there are three invariant points, seven univariant curves, and five metastable crystallization regions corresponding to lithium sulfate monohydrate (Li2SO4 center dot H2O), arcanite (K2SO4), epsomite (MgSO4 center dot 7H(2)O), double salt 1 (LiKSO4), and picromerite (K2SO4 center dot MgSO4 center dot 6H(2)O) formed in the metastable quaternary system. A comparison of the diagrams between the metastable equilibrium at 288.15 K and the stable equilibrium at 298.15 K shows that the metastable behavior of magnesium sulfate is obvious and the crystallizing regions of epsomite and arcanite are much larger than those in the stable phase diagram. The physicochemical properties of the quaternary system change gradually and regularly with the concentration of magnesium sulfate. No solid solution was found.