Аннотация:We present far-infrared reflectivity spectra of Pb0.75Sn0.25 Te single crystals doped with 0.5 at% of indium. The experimental spectra are analysed using a numerical fitting procedure based on the plasmon-phonon interaction model. The plasma frequency is found to decrease on cooling from 300 to 30 K and a sharp drop in omega-p was observed between 30 K and 5 K. If the free carrier distribution over sample thickness is taken into account, experimental and calculated spectra agree well over the above temperature range. The results of galvanomagnetic measurements are also presented.