Fourier transform infrared variable angle spectroscopic ellipsometry (0.089-0.620 eV) was used to investigate undoped and Te doped epilayers of In0.15Ga0.85As0.17Sb0.83 on (100) GaSb substrates. The onset of absorption at the bandgap (0.55 eV) was observed in the data for the undoped epilayer. The data below the bandgap for the doped epilayer displayed free carrier characteristics as well as interference effects. They were simulated by including a Drude term in the epilayer and substrate dielectric functions of a multilayer model. This model produced a good fit to the data and provided the epilayer thickness and free carrier plasma frequency. SE measurements above the gap (0.75-5.4 eV) showed only minor effects due to doping.