The solubilities and physicochemical properties such as density and refractive index of the solution in the ternary systems KCl + MgCl2 + H2O and KCl + RbCl + H2O were investigated at 298.15 K using an isothermal evaporation method. The crystalloid forms of the solid phase were determined using chemical analysis and an X-ray diffraction method. On the basis of experimental data, the metastable equilibrium phase diagrams and the physicochemical properties vs composition in the ternary systems at 298.15 K were constructed. In the metastable phase diagram of the ternary system KCl + MgCl2 + H2O at 298.15 K, there are two invariant points, three univariant curves, and three crystallization fields corresponding to potassium chloride (KCl), magnesium chloride hexahydrate (MgCl2 center dot 6H(2)O), and a double salt carnallite (KCl center dot MgCl2 center dot 6H(2)O). In the metastable phase diagram of the ternary system KCl + RbCl + H2O, there are two invariant points and three crystallization fields corresponding to the single salts potassium chloride (KCl), rubidium chloride (RbCl), and a solid solution of potassium and rubidium chloride [(K,Rb)Cl]. Comparisons between the metastable phase diagrams at 298.15 K and at 323.15 K show that the crystallization forms have not changed, while the crystallization zones have slight changes. The calculated densities and refractive indices using an empirical equation have the maximum relative error < 0.016.