Abstract: The isothermal equilibrium solubility of the ternary system of RbCl-CsCl-H2O was studied at 50 ℃ by the isothermal equilibrium method.The isothermal equilibrium phase diagrams were plotted in line with the experimental data.Research results showed that the phase diagram of RbCl-CsCl-H2O was a simple eutectic type where one saturation point was found to have the compositions of 22.3%RbCl，53.8% CsCl，and 23.9% H2O by mass，respectively.At this time，RbCl and CsCl both achieved the saturation point.The salting-out solid-phase crystal was pure RbCl salt at first，then was RbCl and CsCl two common salt precipitation，and finally was pure CsCl.The study provided fundamental thermodynamic data for salt lake and underground brine，and had a practical significance for comprehensive utilization of the solution chemistry，thermodynamic model，and brine systems that contain rubidium and cesium.

Key words: rubidium chloride, cesium chloride, salt-water system, phase equilibrium