Spectroellipsometry represents a powerful technique in non-destructive and contactless material characterization. In this paper. using the above mentioned spectroellipsometric technique, we measure the refractive index profile of wet thermal SiO2 films on single-crystal silicon substrate under various thermal oxidation conditions. The optimum conditions for an accurate evaluation of the thin films parameters - refractive index and thickness - are determined. The effect of different oxidation conditions upon these parameters are further investigated. (C) 2001 Elsevier Science B.V. All rights reserved.