Abstract: The eqtiibrium solubilities of title quaternary system were studied at 50,75℃ by the isothernlal method. The phase diagrams of the system consist of seven solubility branches and have five crvstallization fields : KLiSO₄, Li₂SO₄·H₂O, KCl,K₂SO₄, and LiCl·H₂O. The first one is the largest. Three invariant points exist in the system : LiCl·H₂O +KCl +Li₂SO₄·H₂O,KCl+K₂SO₄+KLiSO₄,andKCl+KLiSO₄+Li₂SO₄·H₂O. The first one is a congrLient eutonic point, and the others are incongruent. The ion interaction pararneters of the Pitzer model for rnixed electrolyte solution at 50℃ and 75℃ were obtained by fitting the determined solubility data. The water activity of the equilibrium liquid, and the solubility product of saIts were calculated. The empirical eqtiations of density and refractive index of the solution in relationto to its composition were used to correlate the data. The results are in good agreement.

Key words: lithitim salt, potassium salt, chloride, sulphate, equilibrium solubility, phase dia-gram, Pitzer model