We report the longitudinal optical (LO) phonon frequencies for thin films of the ternary alloyCdSxTe1−xand their composition dependence for the full range ofxvalues from pure CdTe to pure CdS. Pulsed laser deposition was used to prepare the polycrystalline thin films including compositions well inside the miscibility gap. We find that this ternary system exhibits a “two-mode” behavior with CdS- and CdTe-like longitudinal optic phonon modes. The modified random-element isodisplacement model yields a good description of the composition dependence of the LO phonon frequencies. ©1997 American Institute of Physics.