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. (C) 2014 Elsevier B.V. All rights reserved.