The study of bulk phonons in thin films is limited due to the low energy of far-infrared sources. The infrared reflection-absorption spectroscopy (IRAS) technique has, therefore, been developed. In the case of incidence deviating from normal two states of polarization of incident light should be taken into consideration. The application of approximation of ultra thin films enables to obtain the simple relations Delta R\R-0 between the change of reflectivity of a metal plate covered with a film and that of the metal substrate. Delta R\R-0 has been obtained for two polarizations, namely: s and p. The present research has demonstrated that this method proved to be particularly usable for the multimode crystals where the oscillator strength is distributed on several modes. The IRAS technique has been applied to the ZnxCdyHg1-x-yTe (ZMCT) thin films. The frequencies of LO and TO phonons were determined by comparing the theoretical and experimental spectra.