The impact of process conditions and crystal properties on the structure of thermal oxides formed on AlN were determined by high resolution transmission electron microscopy (HRTEM). Oxidation for 2 hours at both 800 ° and 1000 °C produced mostly amorphous oxide layers whereas oxidation for 4 and 6 hours at 1000 °C produced partly crystalline and epitaxial oxide layers. The crystalline oxide was mostly single phase á-Al2O3 except at the surface where it was a mixture of γ-Al2O3 and á-Al2O3. The amorphous oxide layer first transformed to γ-Al2O3 and then to the stable á-Al2O3 as evidenced by the non-uniform thickness of the oxide and the existence of the γ-Al2O3 at the surface. The AlN crystal contained a high density of defects at the interface at 800 °C but it was nearly defect- and oxygen-free at 1000 °C. This could be due to the rapid diffusion of the nitrogen and aluminum interstitials at high temperatures leading to a point defect equilibrium throughout the nitride. A faceted interface between Al2O3 and AlN could be attributed to non-uniform out diffusion of aluminum.