We have examined the optical phonons of InSb1-xAsx using oblique-incidence infrared spectroscopy. Reflectance spectra of thick (similar to2 mum) relaxed layers grown on InSb (100) by liquid phase epitaxy exhibit for x<0.65 a weak reflectivity peak near 235 cm(-1) together with a strong substrate peak. No other prominent features were seen. Fitting the spectra with a factorized model yielded the transverse and longitudinal mode frequencies and damping constants. The InSb-like mode frequencies were only weakly dependent on Sb concentration for x<0.65. Although it had been expected that InAs1-xSbx would exhibit one-mode behavior, this proved to be true only for x<0.49 above which concentration two modes were seen. (C) 2004 American Vacuum Society.