The growth of oxide films on two types of aluminium nitride substrates of different origin has been studied as a function of temperature. At a given set of oxidation reaction parameters, the oxide layers grown on substrates with a relatively large grain size and high concentrations of Y-Al-O-based liquid sintering aid phases (type I substrates) were observed to be thicker and more diffuse than those obtained on substrates with an average particle size of approximately 3 μm and low liquid sintering aid concentrations (type II substrates). The effects of the oxygen partial pressure variation on the oxide film growth have been investigated for the oxidation of type II AIN substrates. The kinetics of the growth of oxide films on such substrates were analysed and determined to fit best to a linear rate law. This type of rate law indicates that the rate-limiting step in the growth of oxide films on high-quality type II aluminium nitride substrates is an interface reaction-controlled process.