In this work, the refractive index profiles of SiO2/Si3N4/SiO2/silicon (ONO) structures were measured and analyzed by ellipsometry. The ONO structures were obtained by oxidizing the Si3N4/SiO2/silicon structure in a wet O2 ambient at the temperature range of 900–1050 °C for different lengths of time. It was found that for a nitride film thickness less than 250 Å, oxygen not only oxidized the surface, but diffused through the nitride and oxidized the inner surface of the nitride. This result was confirmed by the Auger analysis. The kinetics of the wet O2 oxidation of nitride at the above temperature range was linear for an oxidation time of 30–120 min. The activation energy was 2.24 eV.