Experimental studies on the metastable solubilities and the physicochemical properties including density, viscosity, refractive index, conductivity, and pH of the aqueous quaternary system (KCl + K2SO4 + K2B4O7 + H2O) at 308.15 K were determined with the method of isothermal evaporation. According to the experimental results, the dry-salt phase diagram, the water diagram, and the physicochemical properties via composition diagrams were plotted. It was found that there is one three-salt cosaturation point of KCl + K2SO4 + K2B4O7 center dot 4H(2)O, three univariant curves, and three metastable crystallization fields corresponding to potassium chloride, potassium sulfate, and potassium borate tetrahydrate formed in the metastable quaternary system. Neither solid solution nor double salts were found.