The molalitiesmof (m1KCl+m2CuCl2)(aq) have been investigated in saturated solutions at the temperatureT=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 (m1KCl+m2NiCl2)(aq), (m1K2SO4+m2NiSO4)(aq), (m1KCl+m2CuCl2)(aq), and (m1K2SO4+m2CuSO4)(aq) have been simulated thermodynamically atT=298.15 K on the basis of the Pitzer 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 ΔrGm° of the synthesis of double salts KCl·CuCl2·2H2O, 2KCl·CuCl2·2H2O, K2SO4·NiSO4·6H2O, and K2SO4·CuSO4·6H2O from the corresponding simple salts, and their standard molar Gibbs energies of formation ΔfGm° have been determined. The basic Pitzer ion-interaction model has been used for determining the thermodynamic characteristics of the formation process of mixed crystals of the double salts K2SO4·NiSO4·6H2O and K2SO4·CuSO4·6H2O. The component activities of the saturated (m1K2SO4·NiSO4· 6H2O+m2K2SO4·CuSO4·6H2O)(aq) and in the mixed crystalline phase are determined, and the change of the molar Gibbs energy of mixing ΔmixGm°(s) of crystals is determined depending on the solid phase composition given in mole fractions. It is established that atT=298.15 K the mixed K2SO4(Ni, Cu)SO4·6H2O crystals show small positive deviations from the ideal mixed crystals.