Thermal nitridation of silicon in ammonia or ammonia‐argon mixtures at temperatures between 900° and 1200°C has resulted in the formation of oxynitride films that are oxidation resistant. Typical film thicknesses were 50–60Å after nitridation for 3–16 hr at 1100°C in pure ammonia. The temperature of nitridation and the ammonia content in the ambient had little effect on the final film thickness. The growth kinetics followed a rapid initial increase for 1–3 hr, with no significant increase thereafter. The nitrogen content of the film increased with increasing ammonia content of the annealing ambient and with the temperature and time of treatment. The nitrogen fraction varied from 0.30 to 0.65 (1.00 for pure silicon nitride). Etch rates of films with were 20–22 Å/min in 7:1 BHF. These films were resistant to dry and wet oxidations at temperatures below 1200° and 1000°C, respectively; for example, a film grown for 40 min in pure ammonia at 1100°C inhibited substrate oxidation for nearly 3 hr at 1100°C in dry oxygen. The oxidation resistance increased with the nitrogen content, and oxidation proceeded in three steps. The first was a rapid substitution of nitrogen with oxygen, until decreased to 0.20. In the second, at , the film was most resistant to oxygen diffusion, and about 70% of the total oxidation resistance time was accounted for by the film in this composition range. In the third, below , again decreased rapidly. The film lost its oxidation resistance near or less.