Sulfur-incorporated nanocrystalline carbon (n-C:S) thin films were grown on molybdenum substrates by hot-filament chemical vapor deposition technique using gas mixtures of methane, hydrogen and a range of hydrogen sulfide (H2S) concentrations (100-500 ppm with an interval of 100 ppm) at a fixed substrate temperature of 900 degreesC. They were optically characterized using ex situ spectroscopic phase modulated ellipsometry from near IR to near UV (1.5-5.0 eV) obtaining their pseudo-dielectric function as a function of [H2S]. The ellipsometry data (, ) were modeled using Bruggeman effective medium theory and dispersion relations for the amorphous semiconductors: Forouhi and Bloomer (FB) parameterization model. A simplified two-layer model consisting of a top layer comprising an aggregate mixture of sp(3) C+sp(2) C+void and a bulk layer (L-2), defined as a dense amorphized FB-modeled material, was found to simulate the data reasonably well. Through these simulations, it was possible to estimate the dielectric function of our n-C:S material, along with the optical bandgap (E-g), film thickness (d) and an overlayer thickness (L-1), which is equivalent to surface roughness layer (sigma(SE)) as a function of H2S concentration. The physical interpretation of the five modeling parameters obtained in the amorphous dispersion model applied to the case of n-C:S thin films is discussed. The optical bandgap was found to decrease systematically with increasing H2S concentration, indicating an enhancement of pi-bonded carbon (sp(2) C), in agreement with RS results. These results are compared to those obtained for films grown without sulfur (n-C), in order to demonstrate the influence of sulfur addition on film microstructure. These analyses led to a correlation between the film microstrucrure and its electronic properties. (C) 2003 Elsevier B.V. All rights reserved.