The mixed thermal oxidation of the high temperature aluminium nitride (HT-AlN) epitaxial layer leads to the formation of the stable aluminium oxide having a developed surface area, potentially suitable for sensor applications. In this work, we studied the process of oxide formation on HT-AlN layers versus oxidation time. We applied scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to perform analysis of the morphology and chemical depth profiles of the oxide/AlN structures obtained at gradually increased time of oxidation. The SEM images revealed porous, semi-crystalline columnar structure of HT-AlN layers and largely developed surface of aluminium oxide films. We recognized two oxidized regions with different features: stoichiometric Al2O3 layer near the top structure surface and partially oxidized, gradually changing Al-oxynitride region between Al2O3 and AlN, without clearly determined boundaries. We estimated the thicknesses of these regions and compared the results with the ellipsometric and energy-dispersive X-ray spectroscopy measurements. We proposed a morphological and chemical model of the oxidized structures, including both the fully oxidized alumina film and oxynitride region adjacent to the AlN region. We found two competitive components of the oxidation process, namely an almost linear Al2O3 growth and gradual decay of the Al-oxynitride layer with oxidation prolongation.