Low‐pressure chemical vapor‐deposited (LPCVD) silicon nitride films on silicon have been characterized by means of Rutherford backscattering (RBS), Auger electron spectroscopy (AES) combined with ion sputtering, and spectroscopic ellipsometry. It appeared that all LPCVD samples in the examined thickness range of 50 –500 A˚ had an oxygen‐containing layer equivalent to 15–20 A˚ of SiO2at the nitride‐silicon interface. This interfacial layer originates from the native silicon oxide present at the silicon substrate when the deposition of nitride is started. For comparison, oxide‐free silicon substrates were nitrided in ammonia at temperatures between 800–1160 °C. The thermal nitride films were found to be very thin, at the most 30 A˚, even after 5 h of nitridation. Both the LPCVD and thermal nitride films oxidize slightly when transferred into the ambient; a surface layer equivalent to 8 A˚ of SiO2was detected. Auger and RBS results agree very well for all nitride films investigated. It is shown that RBS can be made a very sensitive probe for such thin multilayer structures by performing glancing‐angle scattering experiments under channeling conditions. Spectroscopic ellipsometry proves to be more sensitive to the interface structure than to the surface composition. Although quantitatively deviating results were obtained, in the extremely thin thermal nitrides this optical technique traces chemical intermediates, not perceived with RBS or AES.