Abstract In view of their application as oxidation masks in a modified LOCOS technology the oxidation behaviour of LPCVD silicon oxynitrides was investigated. In dry oxygen hardly any oxidation occurs. The oxidation rate increases with partial water pressure in the ambient. However, in any case the oxidation of oxynitride is at least one order of magnitude smaller than that of silicon. Layers with a composition near O/N = 0.4 were found to have the greatest oxidation resistance. The performance can be further improved by annealing the films prior to oxidation. A model is proposed in which the out-diffusion of N-containing reaction products limits the rate of oxidation. Hydrogen, originating from both the ambient and the as-deposited layer, plays a peculiar role in this process.