Interference in an air gap between a reference surface (a glass) and the surface to measure can be used to obtain its reflectance in a robust way and also to obtain the ellipsometric parameter Delta. In the methodology proposed, there is no need to measure a reference beam. From several reflection spectra taken at different air-gap thicknesses and at p and s polarization, we get three magnitudes R-p, R-s and Delta. We describe the technique and demonstrate experimentally its viability. A sample of silicon with a thin layer of thermally grown silica is used for such purpose. (C) 2003 Elsevier B.V. All rights reserved.