Aluminium nitride powders are very sensitive to oxidation: nitrogen atoms are easily substituted by oxygen atoms in the hexagonal structure. Thermoluminescence (TL), in detecting defects in the matter can be used to study oxygen impurities in the AlN lattice. So we tested two powders with different synthesis processes and thermal treatments in order to observe the influence of the oxygen content on the TL curves. First aluminium nitride synthesised with amines and aluminium halides revealed the relation between the TL signal and crystallite size: the TL intensity increased despite the removal of oxide phases and the decrease of oxygen content. Second, Pyrofine R AlN was treated under oxygen flow for 3 h between 200 ° and 1100 °C. The TL spectrum showed the presence of γ-AlON at low temperature and the area of the TL peak (80 °C) increased with the weight ratio of AlN.