The phase equilibrium diagram for the CuI + CsI system has been redetermined by differential scanning calorimetry and X-ray diffraction. The phase diagram exhibits three intermediate compounds-CS(3)CU(2)I(5), melting incongruently at 663.0 K, CSCU(2)I(3), melting congruently at 656.0 K, and CsCu(9)I(10), undergoing a solid-state disproportionation reaction to CuI + CSCU(2)I(3) at 604.8 K. The existence of CsCu(9)I(10), derived from DSC studies in accordance with the phase rule, is not shown by X-ray diffraction investigations.