Thin films of conducting polymer complex poly(3,4ethylenedioxythiophene) : polystyrene sulfonate (PEDOT:PSS) are studied by means of varied angle spectroscopic ellipsometry (VASE) in the wavelength range 370-880 nm, at hve different angles of incidence (AOI). The goal is to simultaneously determine the film thicknesses and its dielectric function. Samples of varied film thickness are prepared by spin coating the aqueous suspension onto Si/SiO₂ wafers at different spin rates, and subsequent annealing at 170 °C. The measured spectra at constant angles of incidence are htted by applying Fresnel's equations for a stratihed optical layers model (SLM) which postulates several optical layers with well dehned optical interfaces, in combination with the dielectric function ε̅(λ). Different possibilities for data htting are critically assessed, pointing to some pitfalls in the VASE data analysis and interpretation. Two methods which involve the assessment of a theoretical Lorentz-Drude (LD) dielectric function were compared: one in which a unique function is assumed for all of the prepared samples (i.e. hlm thicknesses), while in the other, a variation of the dielectric function was permitted. The validation criteria include comparisons with the experimental hlm thickness measured by means of the atomic force microscope, the quasi-experimental dielectric function htted independently for each wavelength, and the literature value of the main optical transition energy calculated by quantum mechanical modelling. In the limit of the studied system and model, all of these comparisons point to the necessity of simultaneously htting the VASE data measured for hlms of varied thickness, in which case both the thickness and the dielectric function may be determined.