SiO2 layers on Si are important for many applications, especially in the semiconductor industry. The thickness of the oxide layer can be determined from interferences in the X-ray reflectance, but these oscillations are very weak due to the low contrast between the optical constants of Si and SiO2 for the Cu Kalpha radiation (8048 eV) typically used. When monochromatized synchrotron radiation with an energy of 1841 eV slightly above the Si K edge is used, pronounced oscillations occur. Oxide layer thicknesses from a few nanometer up to 1 mum can be determined with low uncertainties. (C) 2003 Elsevier B.V. All rights reserved.