An experimental study on metastable equilibria at 288 K in the quaternary system Li^＋, Na^＋//SO₄^2－, CO₃^2－-H₂O was done by isothermal evaporation method. Metastable equilibrium solubilities and physico-chemical properties such as density, conductivity, pH, refractive index and viscosity of the solution were determined experimentally. The experimental results of the metastable equilibria show that the double salt [Na₃Li(SO₄)₂•6H₂O] forms in the quaternary system Li^＋, Na^＋//SO₄^2－, CO₃^2－-H₂O at 288 K. According to the experimental data, the metastable equilibrium phase diagram and the physico-chemical composition diagram were plotted. In the metastable phase diagram, the quaternary system has three invariant points, seven univariant curves, five fields of crystallization [Li₂SO₄•H₂O, Na₂SO₄, Li₂CO₃, Na₂CO₃•10H₂O, Na₃Li(SO₄)₂•6H₂O]. The double salt Na₃Li(SO₄)₂•6H₂O and Li₂SO₄•H₂O have smaller crystallization fields than the others. The crystallization field of Na₂SO₄•10H₂O was not found in the metastable equilibria.

Key words: metastable equilibrium, carbonate, sulfate, solubility