The kinetics of thermal nitridation of silicon dioxide in ammonia ambient has been studied. films of 100–1000Å thick were thermally nitrided at 950°–1100°C for times from 15s to 2h. Our experimental results based on etch rate and Auger electron spectroscopy measurements clearly indicate the multilayer structure of nitrided‐oxide films. Nitrogen‐rich layers are formed at the surface and interface regions at a very early stage of the nitridation process. After a few minutes, the nitridation reaction mainly goes on in the bulk region, with the surface and interface nitrogen content remaining fairly constant. The Auger depth profiles show that the interface moves away from the nitrogen‐rich layer as the nitridation proceeds. In addition, our results indicate the formation of an oxygen‐rich layer underneath the nitrogen‐rich layer whose thickness increases with nitridation time. The formation of this oxide‐like layer can be attributed to a slow oxidation of the silicon substrate at the nitroxide/silicon interface by the oxygen which is a by‐product of the bulk exchange reaction between (or nitrogen containing species) and . The results of this work can be qualitatively used to explain effects such as enhanced boron and phosphorus diffusion and growth of stacking faults in the silicon substrate during nitridation of oxide.