The molalities m of (m(1)CuCl(2) + m(2)CuSO(4))(aq) have been investigated in saturated solutions at the temperature 298.15 K by the physico-chemical analysis method. Only the crystallization of the simple salts CuCl2. 2H(2)O, and CuSO4. 5H(2)O have been established. The ternary solutions (m(1)NaCl + m(2)CuCl(2))(aq), (m(1)Na(2)SO(4) + m(2)CuSO(4))(aq), and (m(1)CuCl(2) + m(2)CuSO(4))(aq) have been simulated thermodynamically at T = 298.15 K using the Pitzer model. The ternary parameters of ionic interaction have been chosen on the basis of the compositions of saturated ternary solutions taking into account the unsymmetrical mixing terms. The calculated thermodynamic properties have been used for a thermodynamic study of the quaternary reciprocal system Na-Cu-Cl-SO4-H2O. A very good agreement is found between calculated and experimental solubility isotherms.