Thin aluminium oxynitride films are formed by plasma nitridation of aluminium. The process is studied for anodic and cathodic treatment in rf and dc generated plasmas of pure nitrogen. The most effective process regarding large thickness and low oxygen content of the films is the cathodic dc nitridation. At typical conditions (325 °C, 130 Pa gas pressure, and 30 min duration) the film thickness reaches 12.5 nm and the oxygen content is 7%. The films are homogeneous and metal‐free but show a graduated transition to the underlying unreacted aluminium metal. The nitrogen transport mechanism is diffusion controlled but influenced by electron bombardment (anodic nitridation) and ion implantation (cathodic nitridation), respectively.