The phase equilibria and phase diagrams are very helpful in describing the geochemical behaviors of aqueous solution and provide great guidance for the separation of salts. In this paper, solid-liquid phase equilibrium for the reciprocal quaternary system (Na+, Cs+//Cl-, SO42--H2O) at T = 298.15 K and 0.1 MPa was carried out with an isothermal dissolution method. The solubilities and physicochemical properties (densities and refractive indices) in this reciprocal quaternary system (Na+, Cs+//Cl-, SO42--H2O) were investigated. The dry-salt and water-phase diagrams, as well as physicochemical properties versus composition diagrams were also established. Three invariant points, eight univariant curves, and six crystallization regions corresponding to cesium sulfate (Cs2SO4), halite (NaCl), thenardite (Na2SO4), mirabilite (Na2SO4 center dot 10H(2)O), a solid solution aCs(1-x)(Na center dot H2O)(x)]Cl), and a double salt (CsCl center dot 2NaCl center dot 2H(2)O) could be found in this system. The crystallization region of thenardite was the largest, indicating thenardite could be easily crystallized and separated from this system. In addition, the physicochemical properties showed a regular change with the increase of Janecke index values of J(Na-2(2+)).