Polycrystalline thin films deposited by coevaporation of CdTe and CdS form metastable single-phase CdTe-CdS alloys. Subsequent heating in a kinetic-enhancing ambient segregates CdTe1-xSx, and CdS1-yTey. phases from the original alloy phase. The equilibrium miscibility gap between the resulting CdTe1-xSx and CdS1-yTey alloy phases is determined for CdTe1-xSx films with initial x similar to 0.4 treated front 360 to 700 degreesC. At 625 degreesC, the equilibrium compositions correspond to published results for mixed crystals. Below 625 degreesC the miscibility gap widens asymmetrically due to different mixing free energies for S in CdTe and Te in CdS. The solubility thermodynamics are modeled with an excess mixing free energy to account for nonideal mixing behavior.