A detailed study of the refractive indices of the narrow-band-gap III–V semiconductors is presented. The calculation is based on a numerical Kramers–Kronig analysis of the carrier-related imaginary part of the dielectric function. The near-band-gap refractive index spectra of InSb, InAs, and GaSb for different free-carrier densities are obtained. The density dependence of the refractive index is analyzed and a comparison with the results from the Drude theory is performed. The refractive indices of In1−xGaxSb and InAsxSb1−xternary alloys are also investigated. The obtained results are expected to be useful in the design optimization of the phase modulators and the laser diodes for midinfrared applications. ©1997 American Institute of Physics.