The metastable phase equilibria in the aqueous quaternary system LiCl + KCl + RbCl + H2O were investigated at 298.15 K using an isothermal evaporation method. The solubilities and physicochemical properties such as the density and refractive index of the equilibrated solution were determined. On the basis of the experimental results, the stereo phase diagram, the projected phase diagram, the water content diagram, and the physicochemical properties versus composition diagrams were constructed. The projected phase diagram consists of two invariant points, five isothermal univariant curves, and four crystallization fields corresponding to single salts potassium chloride (KCl), rubidium chloride (RbCl), lithium chloride monohydrate (LiCl center dot H2O), and a solid solution of potassium and rubidium chloride [(K, Rb)Cl]. The two invariant points are cosaturated with three salts, which are LiCl center dot H2O + RbCl + (K, Rb)Cl and LiCl center dot H2O + KCl + (K, Rb)Cl, respectively. No double salt was formed. The crystallization region of the solid solution [(K, Rb)Cl] almost occupies the entire phase region, which shows that it is difficult to separate potassium from rubidium in chloride solution by only using evaporation and crystallization methods at 298.15 K. The calculated densities and refractive indices using empirical equations have a maximum relative error of <0.016.