The molalities m of(m(1)KCl + m(2)CuCl(2))(aq) have been investigated in saturated solutions at the temperature T = 298.15 K by the physico-chemical analysis method. The experimental data obtained are in very good agreement with those available in the literature. The solutions (m(1)KCl + m(2)NiCl(2))(aq), (m(1)K(2)SO(4) + m(2)NiSO(4))(aq), (m(1)KCl + m(2)CuCl(2))(aq), and (m(1)K(2)SO(4) + m(2)CuSO(4))(aq) have been simulated thermodynamically at T = 298.15 K on the basis of the Fitter model. The necessary thermodynamic functions (binary and ternary ion-interaction parameters, thermodynamic solubility products) have been calculated, and the theroretical solubility isotherms plotted. The calculated data are in very good agreement with the literature data as well as our experimental data. The standard molar Gibbs energies of reaction Delta(r)G(m)degrees of the synthesis of double salts KCl . CuCl2 . 2H(2)O, 2KCl . CuCl2. 2H(2)O, K2SO4. NiSO4. 6H(2)O, and K2SO4. CuSO4. 6H(2)O from the corresponding simple salts, and their standard molar Gibbs energies of formation Delta(f)G(m)degrees have been determined. The basic Fitter ion-interaction model has been used for determining the thermodynamic characteristics of the formation process of mixed crystals of the double salts K2SO4. NiSO4. 6H(2)O and K2SO4. CuSO4 . 6H(2)O. The component activities of the saturated (m(1)K(2)SO(4). NiSO4. 6H(2)O + m(2)K(2)SO(4). CuSO4. 6H(2)O)(aq) and in the mixed crystalline phase are determined, and the change of the molar Gibbs energy of mixing Delta(mix)G(m)degrees(s) of crystals is determined depending on the solid phase composition given in mole fractions. It is established that at T = 298.15 K the mixed K2SO4(Ni,Cu)SO4. 6H(2)O crystals show small positive deviations from the ideal mixed crystals.