Solid-liquid equilibria of the quaternary system Cd2+, K+, Na+//SO42--H2O at 298 K were studied by an isothermal solution saturation method. Experimental results indicate five univariant curves E1F1, E1F4, E1E2, E2F2 and E2F3; two invariant points E-1 and E-2; four crystallization fields; and the existence of a double salt (Na2SO4 center dot 3K(2)SO(4)) in the quaternary system. Crystallization fields of equilibrium solid phases are K2SO4 (E1F1MF4 field), Na2SO4 center dot 3K(2)SO(4) (E1E2F3F4 field), Na2SO4 center dot 10H(2)O (E(2)F(2)AF(3) field), and CdSO4 center dot H2O (E2E1F1BF2 field). The composition of invariant point E-1 is K2SO4, Na2SO4 center dot 3K(2)SO(4) and CdSO4 center dot H2O. The composition of invariant point E-2 is Na2SO4 center dot 3K(2)SO(4), Na2SO4 center dot 10H(2)O and CdSO4 center dot H2O. Physico-chemical properties of solution in the quaternary system show regular changes with increased cadmium concentration, indicating that CdSO4 center dot H2O possesses the highest solubility among the three salts. This result verifies a strong transfer of Cd ion and a potential risk of soil pollution. It further shows that the solubility of K2SO4 would be restrained when the salts coexist. (C) 2013 Elsevier B.V. All rights reserved.