The components (solubilities and densities) in aqueous stable equilibria systems Cs2SO4 + MgSO4 + H2O and Rb2SO4 + Cs2SO4 + H2O at T = 298.2 K have been studied by the isothermal dissolution equilibrium method. The wet residue method and the X-ray diffraction method were used to confirm the solid phase compositions. The ternary system Cs2SO4 + MgSO4 + H2O is a complex saturated type, with the commensurate double salt Cs2SO4 center dot MgSO4 center dot 6H(2)O formed at the investigated temperature. There are two two-salt cosaturation invariant points, with the coexisting solid phases corresponding to commensurate type saturation points E-1 (MgSO4 center dot 7H(2)O + Cs2SO4 center dot MgSO4 center dot 6H(2)O) and E-2 (Cs2SO4 center dot MgSO4 center dot 6H(2)O + Cs2SO4), three monovariant curves, and three crystallization fields in the system of Cs2SO4 + MgSO4 + H2O. The ternary system Rb2SO4 + Cs2SO4 + H2O with solid solutions formed contains two cosaturated points, with the equilibrium solid phases of the saturation points F-1 and F-2 corresponding to [(Rb, Cs)(2)SO4] + Cs2SO4 and Rb2SO4 + [(Rb, Cs)(2)SO4], respectively, three monovariant curves, and three crystallization fields and is also a complex type at 298.2 K. Furthermore, under the investigated temperature, the Pitzer model was applied to calculate the solubilitites of these two ternary systems. Comparing the predicted solubilities with the experimental solubilities, the results both indicated good consistency.