We propose a method to analyze reflectance and transmittance spectra of hydrogenated amorphous silicon-rich carbide alloys, which allows the determination of n–k spectra with very good accuracy. The method is based on the Tauc–Lorentz formulation proposed by Jellison and Modine, in which we introduce an absorption band to take into account the tail states. The method is used to retrieve the n–k spectra of a set of silicon-rich carbide films fabricated in a variety of conditions. The results on oscillator parameters are discussed. We show that by introducing this absorption band we achieve a better accuracy in the determination of the oscillator parameters. The resulting oscillator frequency appears to be well correlated with compositional data.