Solid liquid equilibria on the reciprocal quaternary system (Na+, K+//Cl-, SO42--H2O) and its ternary system (Na+,K+//SO42--H2O) at 313.15 K are studied with the method of isothermal solution saturation. The solubilities and physical properties (n(D), eta, rho) of saturated solutions in the two systems are determined. The phase diagram of the ternary system at 313.15 K includes two invariant points, three invariant curves, and three fields of crystallization of single salts corresponding to K2SO4, Na2SO4 center dot 3K(2)SO(4), Na2SO4. The two invariant points are cosaturated with two salts, that is (K2SO4 + Na2SO4 center dot 3K(2)SO(4)) and (Na2SO4 + Na2SO4 center dot 3K(2)SO(4)), respectively. The reciprocal quaternary system at 313.15 K is divided into seven univariant curves, three invariant points cosaturated with, three-salts and five crystallization fields. A double salt Na2SO4 center dot 3K(2)SO(4) can be proven to exist in the reciprocal quaternary system (Na+, K+//Cl-, SO42--H2O) and the ternary system (Na+, K+// SO42--H2O) under phase equilibrium conditions, at 313.15 K. On the basis of the phase diagram, the crystallization field of GLa and K2SO4 is bigger than that of the other three salts, whereas the KCl crystallization field is the smallest. Therefore, K2SO4 can crystallize out more easily from this mixing equilibrium solution except for Na2SO4 center dot 3K(2)SO(4). The experimental results showed that both systems are a complex eutectic type, and no solid solutions are found at the investigated temperature. All the physical properties (n(D), eta, rho) of the equilibrium quaternary system and ternary system change regularly with concentration change of the liquid phase. The measured data and phase equilibrium diagrams can be used for a new technology to design and optimize the separation process of K2SO4 (KH2PO4), and provide fundamental data support for chemical industry development.