As potential anti-oxidation coatings, several AlN-Si3N4 codeposits have been grown by chemical vapour deposition at low pressure (LPCVD) in a vertical hot-wall reactor on silicon carbide (SIC) layers. Transmission electron microscopy (TEM) studies have been performed in order to determine both the microstructure and local chemical composition of these deposits. It has been confirmed that the material consists of a mixture of aluminium nitride (AlN) and silicon nitride (Si3N4) phases, whose proportion and crystallization are strongly dependent on the reactor temperature and the reactive flow. In particular, the size of the AIN crystals varies from several nanometres to several hundred nanometres. In this last case, structural defects such as dislocation, stacking faults or grain boundaries have been characterized by high-resolution electron microscopy (HREM) imaging.