Silicon oxynitride films, prepared by post-annealing of pre-oxidized silicon wafers in nitric oxide (NO), were characterized by grazing-emission X-ray fluorescence spectrometry (GEXRF). GEXRF is a new method of instrumental analysis that not only allows determination of the film composition, but also provides information on depth distributions and densities of films. Experiments were carried out using a prototype laboratory GEXRF instrument. It was found that nitrogen is predominantly incorporated in the ultra-thin oxide films (similar to 3.5 nm) at the SiO2/Si interface. The measured total amount of nitrogen in the films was around 10(15) atoms/cm(2) and increased with annealing time. A slight decrease in the amount of oxygen with annealing time was observed. Oxygen could be determined with a relative precision of 3%, whereas instrumental improvements were found to be required to determine nitrogen with comparable precision. On selected samples, cross-analysis with Rutherford backscattering (RBS) and Auger electron spectroscopy (AES) was performed.