Silicon dioxide and silicon oxynitride thin films deposited on crystalline silicon substrates were characterized by phase modulated spectroscopic ellipsometry in the UV-visible range. The thickness and the refractive index of the films were calculated by assuming an ambient/film/substrate system with three different approaches : modeling the dielectric function with Cauchy's dispersion formula, using the Bruggeman effective medium approximation and using a direct inversion algorithm method. The comparison of the results obtained by these approaches along with the subsequent qualitative study of possible weak absorption and depth inhomogeneities in the samples allows us to evaluate the accuracy of the experimental data as well as the reliability of the model and results.