Fourier transform infrared spectroscopy measurements are performed in the wavelength range 1.5–6.8 µm (photon energies of 0.8–0.2 eV) to investigate the optical absorption properties of GaSbBi layers, grown by liquid phase epitaxy. Band edge absorption in the material occurs at 0.66 eV indicating a Bi-induced band gap reduction of 60 meV from the room temperature band gap value of 0.72 eV for GaSb. Strong optical absorption is observed for photon energies below 0.5 eV. Theoretical models involving inter valence band transitions in the material is successfully used to explain the mechanism of the observed below band gap absorption in the same. © 2016, Springer Science+Business Media New York.