To understand the crystallographic forms of salts in sulfate systems containing sodium and rubidium (cesium), the stable phase equilibria of ternary systems Na2SO4 + Rb2SO4 + H2O and Na2SO4 + Cs2SO4 + H2O at 298.2 K were studied with the isothermal dissolution method. The composition and crystallographic forms of the equilibrated solid phase was verified by Schreinemaker's wet residue method combined with X-ray diffraction. Experimental results show that there is no double salt or solid solution formed. The stable phase diagram of Na2SO4 + Rb2SO4 + H2O at 298.2 K is characterized by one invariant point (H), two univariant curves, and two crystallographic forms (corresponding to Na2SO4 center dot 10H(2)O and Rb2SO4). The ternary system Na2SO4 + Cs2SO4 + H2O is of the hydrate type II, there are two invariant points (E-1, E-2), three univariant curves, and three crystallographic forms (corresponding to salts Na2SO4 center dot 10H(2)O, Na2SO4 and Cs2SO4). Affected by the unique structure of Cs+, both salt Na2SO4 center dot 10H(2)O and Na2SO4 have been found in Na2SO4 + Cs2SO4 + H2O system at 298.2 K. Furthermore, the density and refractive index of the equilibrated solution possess the analogous variation tendency and regularly change with the change of concentration.