The solubility of the RbCl-CsCl-H2O ternary system has been investigated at the temperature range from 298.15 to 348.15 K by two different methods, namely, the Flow-Cloud-Point (FCP) method and the Isothermal Dissolve Equilibrium (IDE) method. The measured solubility for this ternary system at 298.15 K by FCP method is consistent with the reported result in literature. The measured solubility results for this ternary system at 323.15 K by the FCP and IDE, two different methods that are consistent with each other, which indicated the results are more reliable. Meanwhile, the measured liquid-wet solid-solid lines proved that the equilibrium solid phases are two types of solid solutions (Rb,Cs)Cl-(ss) and (Cs,Rb)Cl-(ss) rather than pure salts (RbCl and CsCl) reported in literature. For the first time, the solubility of the RbCl-CsCl-H2O ternary system at 308.15 and 348.15 K has been measured by IDE method. The measured results shown that the equilibrated solid phases in this ternary system at 308.15 and 348.15 K are two types of solid solution (Rb,Cs)Cl-(ss) and (Cs,Rb)Cl-(ss), at RbCl rich side and CsCl rich side, respectively.