The optical constants, such as the complex dielectric function, complex refractive index, absorption coefficient, and normal‐incidence reflectivity, of Pb1−xSnxTe alloys have been determined using spectroscopic ellipsometry in the 1.15–5.5 eV photon‐energy range at room temperature. The dielectric‐function spectra [&egr;(E)] and their second‐derivative spectra [d2&egr;(E)/dE2] were analyzed using a simplified model of the interband transitions. The composition dependence of the critical‐point parameters and high‐frequency dielectric constant has been obtained from these analyses. ©1996 American Institute of Physics.