The oxidation kinetics of polycrystalline aluminium nitride substrates in air at temperatures in the range 1150 to 1750 degrees C have been studied by measuring the weight increase in the oxidized samples. At the lowest temperature, the oxide layer was not continuous on the AIN surface and the oxidation kinetics followed a linear rate law with an activation energy of 175 kJ mol(-1). At all the higher temperatures, the growth kinetics followed a parabolic rate law with an activation energy of 395 kJ mol(-1). Samples oxidized at these higher temperatures were covered with a dense oxide layer having a fine-grained microstructure.