Optical properties of pyrite thin films obtained by iron sulfuration have been studied in samples of different iron thickness (0.12–0.5 &mgr;m) sulfurated at different temperatures in the range 620–770 K. Dispersion values of the optical absorption coefficient and refractive index for wavelengths 0.8–3.0 &mgr;m have been determined by fitting the optical transmission spectra in this range using a multivariable nonlinear program of optimization. Values of absorption background, optical energy gap, and optical film thickness have been obtained for all the samples. These values have been related to microstructural and electrical properties measured in the films. The background absorption is associated with the polycrystalline nature of the samples, as well as the presence of an interface between pyrite film and glass substrate. The optical energy-gap variation found in the samples is attributed to the difference in structure and stoichiometry in the grain and at grain boundaries, respectively. ©1997 American Institute of Physics.