It is feasible to investigate the thickness of oxide layers on silicon wafers by X-radiation in the 0.1-10 nm thickness range. For example, X-ray photoelectron spectroscopy (XPS) is a well applicable technique, with information depth of a few nm. Fig. 1 presents the principle of this method. An impinging characteristic X-radiation hν (e.g. Al Kα) count rate ejects Si 2p photoelectrons from the Si-substrate (d), with count rate n2, which, on their way to the electron spectrometer, have to pass through the SiOx-interface (c), the SiO2-layer (b) and the contamination overlayer (a), whereas Si 2p photoelectrons ejected from the SiO2-layer, with count rate n2 have just to penetrate the contamination overlayer. The Si 2p electrons originating from the SiOx-interface, for the situation shown in Fig. 1, can be added to the substrate count rate.