The solubilities of electrolyte mixtures ternary systems KCl + RbCl + H2O and KCl + MgCl2 + H2O at 348.15 K were obtained by isothermal evaporation method. The corresponding physicochemical properties of the solution such as density and refractive index were also determined. The Scherinemakers' wet residue method was used to determine the composition of the solid phase. The metastable phase diagram, density versus composition diagram, and refractive index versus composition diagram were constructed on the basis of the experimental data. Results showed that these two ternary systems were both complex type. The incommensurare double salt carnallite (KCl center dot MgCl2 center dot 6H(2)O) was found in the KO + MgCl2+ H2O system at 348.15 K. Comparisons between the metastable phase diagrams of KCl + MgCl2+ H2O system at (298.15, 323.15, and 348.15) K show that the crystallization zone of carnallite decreases with the increase in temperature. The solid solution [(K,Rb)Cl] was formed in the Ka + RbCl + H2O system at 348.15 K. Comparisons between the metastable phase diagrams of KCl + RbCl + H2O system at (298.15, 323.15, and 348.15) K show that the crystallization zone of [(K,Rb)Cl] decreases with the increase in temperature. The empirical equation was used to calculate the refractive indices of the equilibrated solution. The calculated results keep in line with experimental values with the maximum relative error less than 0.005.