X‐ray photoelectron spectroscopy has been used to study the composition of 100‐A˚ thermally grown SiO2films that have been thermally nitrided in ammonia. The SiOxNy/Si interface was studied both by chemical depth profiling of the oxynitride and by removal of the Si substrate with XeF2. It is found that N is distributed throughout the film, but with the concentration higher at the surface and in a region centered 25 A˚ from the film/substrate interface. The interface region itself is found to be oxygen‐rich relative to the rest of the film. Possible models which can explain these results are discussed.