The accurate determination of the dielectric function and thickness of thin films by means of spectroscopic ellipsometry becomes increasingly difficult or even impossible if the layer thickness approaches the nano- or sub-nanometre range. In the case of in-situ ellipsometric process monitoring, where rough and instable experimental conditions may pose a severe problem, errors introduced by windows and less accurate sample alignment make the data evaluation even more complicated. If the experimental setup cannot be improved and therefore errors are difficult to correct, we find the almost forgotten analytic model introduced by Drude [P. Drude, Ann. Phys. 279, 126 (1891)] to be useful to detect errors and artefacts in the commonly used regression analysis. Here we present the extension of Drude's analytical three phase model for the case of slightly absorbing materials. We show its applicability using the example of the surface modification of a ZnO bulk single crystal under vacuum conditions. As only a few spectral features of the ellipsometric spectrum are used in the analysis, the obtained results are subject to more noise compared to conventional regression analysis. We show that it is useful to apply both complementary techniques, the regression analysis and the extended Drude model, for the analysis of thin layers, especially in the case of in-situ ellipsometry monitoring during growth or modification of thin layers respective surfaces. (C) 2014 Elsevier B.V. All rights reserved.