The stable phase diagram of quaternary water-salt system Li+, Na+, Cs+//SO42--H2O was studied at T = 298.2 K by the isothermal dissolution equilibrium method. The solubility and physicochemical properties (density and refractive index) of the equilibrium solution were measured. Moreover, the composition of the solid phase was identified using the X-ray diffraction method. There are seven solids formed in this system at research temperature, including four single salts and three double salts, which are lithium sulfate monohydrate (Li2SO4.H2O), sodium sulfate (Na2SO4), sodium sulfate decahydrate (Na(2)SO(4)10H(2)O), cesium sulfate (Cs2SO4), and double salts Li2SO4 center dot Cs2SO4, 3Li(2)SO(4). Cs2SO4 center dot 2H(2)O, and 3Na(2)SO(4)-Li2SO4 center dot 12H(2)O, respectively. Thus, the stable phase diagram of the quaternary water-salt system contains 5 invariant points, 11 univariant curves, and 7 crystallization regions. The size order of the salts' crystalline phase region is Li2SO4 center dot Cs2SO4 > 3Na(2)SO(4)center dot Li2SO4-12H(2)O > 3Li(2)SO(4). Cs2SO4 center dot 2H(2)O > Na2SO4 center dot 10H(2)O > Li2SO4H2O > Na2SO4 > Cs2SO4. Obviously, the double salts have larger crystallization fields, which means that in the sulfate system coexisting with lithium, sodium, and cesium, double salts are relatively easier to form than a single salt of various components. As a result, lithium and cesium are scarcely possible separated as Li2SO4 and Cs2SO4 from this system at 298.2 K.