Abstract: Isothermal dissolution method was employed to study the ternary system Rb⁺, Mg²⁺//SO₄^2--H₂O at 298 K, the composition and density of the equilibrium liquid phase as well as the composition and existing form of the equilibrium solid phases were determined. According to the experimental data, the stable phase diagram and the density versus composition diagram in the ternary system were plotted. The results show that the ternary system Rb⁺, Mg²⁺//SO₄^2--H₂O at 298 K is a complex one in which the double salt Rb₂SO₄·MgSO₄·6H₂O is formed. The stable phase diagram contains two invariant points (commensurate type saturation points), three univariant curves, and three crystallization fields corresponding to single salt MgSO₄·7H₂O, Rb₂SO₄ and commensurate double salt Rb₂SO₄·MgSO₄·6H₂O. Moreover, the density of the solution in the ternary system was calculated using the classical equation, and the calculated value shows good agreement with the experimental value with the relative error less than 1%.