This paper aims to demonstrate the high capability of infrared spectroscopic ellipsometry (IRSE) for the characterization of very thin organic films and the organic to inorganic interfaces. It is shown that the detection limit of IRSE facilitates the investigation of ultrathin nitrobenzene (NB) films with monolayer sensitivity. This accounts for substrates from semiconductors to metals. The process of reoxidation of a NB terminated silicon (001) surface is also reflected in the infrared ellipsometric parameters and evidently proceeds despite the organic layer. As a complementary method, x-ray photoelectron spectroscopy (XPS) measurements were performed. (c) 2005 American Vacuum Society.