Far infrared reflectivity spectra of polycrystalline In1-xGaxSb were measured and numerically analyzed using the classical dispersion formula and also a fitting procedure based on the modified plasmon-phonon interaction model in the temperature range from 10 K to 300 K. Optical parameters were calculated and discussed. A local mode belonging to the GaSb rich end and two-mode behavior were observed at low temperatures. (C) 2010 Elsevier B.V. All rights reserved.