By employing the method of isothermal evaporation, the metastable phase equilibria of the quaternary systems KCl + RbCl + MgCl2 + H2O were researched at 323.15 K. In this paper, the solubilities and physicochemical properties, such as refractive indices and densities of the equilibrated solution, were determined. Through analysis of the experiments data, the metastable phase diagram, the water content diagram and physicochemical properties versus composition diagrams of the quaternary system were plotted. The study results indicated that in the metastable phase diagram, there were four invariant points (H-1, H-2, H-3, H-4), nine univariant curves, and six crystallization fields. And the six crystallization fields were magnesium chloride hexahydrate (MgCl2 center dot 6H(2)O) and potassium chloride (KCl) and rubidium chloride (RbCl) and a rubidium and magnesium chloride double salt named carnallite (RbCl center dot MgCl2 center dot 6H(2)O) and a potassium and magnesium chloride double salt named carnallite (KCl center dot MgCl2 center dot 6H(2)O) and a solid solution of potassium and rubidium chloride [(K, Rb)Cl]. The solid solution [(K, Rb)Cl] had the largest crystallization field. This showed that only by using evaporation and crystallization methods at 323.15 K, was it difficult to separate potassium from rubidium in chloride solution. The physicochemical properties of the quaternary system change regularly with the changes of composition in aqueous solutions. (C) 2013 Elsevier B.V. All rights reserved.