Abstract Lattice softening of the CdS LO phonon mode was observed by resonant Raman spectroscopy (RRS). This phenomenon was interpreted in terms of the combined effects of grain size and low-dimensionality of the film. As the thickness of the film decreases below 800 Å, the influence of the free-surface on the vibrational spectrum becomes more prominent. Higher order harmonics of the LO-mode also are observed. The intensity of the Raman line is enhanced as it approaches the maximum of the fluorescence emission peak.