As part of the systematic research on the phase stability of alkali halides, the phase diagram of the CsCl-RbCl system was determined by using differential scanning calorimetry (DSC) with the help of high-temperature X-ray diffractometry. It is indicated from the phase diagram that the CsCl-enriched solid solution which possesses rocksalt structure and the RbCl-enriched solid solution which also possesses rocksalt structure separate into two phases below 803 K. It is stressed that the elimination of the thermal hysteresis by annealing is important in the precise determination of phase diagram of this system.