It is demonstrated that powerful laser radiation causes changes in absorbance spectra of an epoxy resin, polyethylene and polysulfone. Thin polymer films were located between infrared AgBrCl optical fibers and exposed to the radiation of a CO2 laser. The output of the laser source was varied in the range 0-8.5 W. Absorbance spectra were measured by FTIR spectrometry. It was found that characteristic absorbance peaks of polymers decay under the powerful IR light. The apparent dependence of peak magnitude on IR radiation power has been established. The mathematical theory of the observed effect is presented. We suggest that the effect under discussion is caused by the oxygen-free thermal action of IR radiation on the chemical structure of the polymers. We established that the phenomenon of peak disappearance with both polymers - thermoplastic (including such engineering polymer as polysulfone) and thermosetting - is of a threshold nature. The proposed method is a valuable instrument for the study of degradation processes in polymers.