The encapsulating properties of 800–1500 Å aluminum oxynitride (AlOxNy) films, deposited on GaAs by low energy ion beam sputtering, were studied over a range of y from 0.1 to 0.8. Particular attention was given to chemical and sputter cleaning procedures. The structures were characterized by optical microscopy, electrical conductivity, Auger profiling, and ellipsometry. The better films were found to withstand annealing to above 900°C with minimal physical deterioration. The films with a higher proportion of oxygen allowed some oxygen diffusion; those made with inferior cleaning procedures allowed an out‐diffusion of arsenic.