Metastable equilibrium solubilities and properties such as densities, conductivity, pH, refractive index, and viscosity of the solution were determined experimentally. According to the experimental data, the metastable equilibrium phase diagram was plotted. In the phase diagram, there are three invariant points, seven univariant curves, five fields of crystallization: Li2SO4 center dot H2O, K2SO4, Li2B4O7 center dot 3H(2)O, K2B4O7 center dot 4H(2)O, and K2SO4 center dot Li2SO4. The double salt K2SO4 center dot Li2SO4 was found in the quaternary system metastable equilibria. Lithium sulfate (Li2SO4) has the highest concentration and strong salting-out effects on other salts. Also, the relationship diagram between the properties and the ion concentration of solution was constructed. It can be seen from the relationship diagram that the equilibrium solution density values, viscosity values, and refractive index values are increased apparently with the rise of sulfate ion concentration, reaching the maximum values at eutonic point F3. Electrical conductivity values and pH values, however, fall down with the rise of ion concentration on the whole. (C) 2005 Elsevier Ltd. All rights reserved.