Investigation of the optical properties of CdS,-, Sex films in the long-wave IR region of the spectrum has revealed resonance between the gap oscillations of selenium atoms and the interference modes of the films. This resonance leads to a sharp increase of the absorption at the frequencies of the gap oscillations. The experimental results agree well with the theoretical calculations. It is also shown that this resonance should always take place when the interference mode coincides with the weakly absorbing IR active oscillations, including those on the low-frequency wing of the dipole oscillation in the region of the anomalous dispersions of the dielectric constant of the layer.