Phase modulated spectroscopic ellipsometry measurements were performed in supercritical CO2 using a Mueller matrix approach allowing the explicit assessment of the cell windows effect on the measurements. The adsorption of CO2 on thermal SiO2 and the swelling of PMMA thin films on a silicon substrate were studied from atmospheric pressure up to 13.89 MPa at 35 and 50 degrees C. A systematic analysis of the deviations between experimental and calculated spectra reveals the presence of a temperature gradient in the vicinity of CO2's critical point where large density fluctuations are present. The temperature gradient produces a graded CO2 refractive index resulting in a pressure-dependent angle of incidence. This mirage effect is analyzed with a simple model that considers the equation of state for CO2 allowing the evaluation of the temperature gradient. Our results are intended to provide a better understanding of spectroscopy ellipsometry studies in supercritical CO2. (C) 2012 Elsevier B.V. All rights reserved.