Non-stoichiometric silicon nitride (SiN) layers were grown on silicon by Plasma Enhanced Chemical Vapor Deposition (PECVB) at temperatures in the range from 300 degrees C to 350 degrees C. After a densification process, explained in the text, these layers were oxidized in a dry oxygen ambient at 1100 degrees C for different periods. Then, a study of the rate of oxidation was made by means of the Auger spectroscopic analysis of oxidized films. It is shown that the SiN oxidation follows a parabolic law similar to the oxidation of silicon or stoichiometric silicon nitride (Si3N4) obtained by other methods. The quality of the SiN films was correlated to that of the silicon oxide obtained from them by determining the absorption coefficient in the infrared (1070 cm(-1)). It was observed that IR absorption due to oxidized SiN coincides with the one corresponding to the thermal silicon oxide with good dielectric quality.