It is demonstrated that the use of X-ray specular reflectivity (XRSR) to measure the geometrical thickness of thin layers allows one to overcome the thickness-optical property correlation in ellipsometry. In addition, a quasi-achromatic compensator is fitted to a rotating polarizer ellipsometer allowing the measurement of cos Delta with an accuracy better than 10(-4) for values close to one, over the entire 0.25-1.7-mu m spectral range. This set up is applied to the study of transparent materials such as Ion Beam Sputtering (IBS) of thin SiO2 layers deposited on silica glass substrates. Thus, a transition layer at the top surface of the substrate has been detected through the cos Delta measurements. Ellipsometric results are in very good agreement with those obtained by photogoniometry. The knowledge of the optical properties of IBS SiO2 leads to a complete understanding of the ellipsometric response of a TiO2/SiO2 mirror.