The electrical properties of thin films of SiO2 can be improved by nitridation and reoxidation. They are very sensitive to the nitrogen profile, as well as to other impurities, such as hydrogen or water, for example, which makes it necessary to carefully adjust processing parameters. The purpose of this work is to characterize very thin oxynitride films produced by rapid thermal processing (RTP) and to optimize their electrical properties in view of applying them in electrically erasable programmable read‐only memories (EEPROMs). We propose a novel etch‐back and test oxidation technique that is very sensitive to traces of nitrogen at the oxynitride/Si interface. We show a strong correlation between the oxidation resistance of the interface and the electrical properties of metal‐oxide‐silicon (MOS) capacitors fabricated with these films. Nitridation conditions that give a maximal resistance to oxidation (for example, 4 s at 1100 °C) also provide the lowest values of interface charges and traps after reoxidation. Using these processing parameters, we achieve a significant increase in charge to breakdown.