The microstructural development of an AlN/Ti diffusion couple, annealed at 1000°C in an argon atmosphere for 0.1-36 h, was investigated using analytical scanning electron microscopy and transmission electron microscopy. The decomposition and diffusion of Al and N atoms into Ti gave rise to various reaction layers at the interface. A δ-TiN layer was initially formed in the reaction zone between AlN and Ti, and the α2-Ti 3Al layer subsequently developed between δ-TiN and Ti. Then an intergranular τ1-Ti3AlN phase was formed in the δ-TiN layer with the orientation relationships τ1 - Ti3 AlN//δ - TiN and (110) τ1 - Ti3 AlN//(110) δ - TiN. The further diffusion of N atoms into the α2-Ti3Al layer led to the growth of δ-TiN and a twinned α2-Ti3Al(N) solid solution, wherein N atoms went to one of the octahedral interstitial sites in an orderly manner upon cooling, resulting in the formation of τ1-Ti3AlN. The orientation relationships between τ1-Ti3AlN and α2-Ti3Al(N) were τ1 - Ti 3 AlN// α2 - Ti3Al (N) and (011)τ1 - Ti3AlN//(1120) α2 - Ti3Al(N). Finally, diffusion paths are proposed for the interfacial reactions at various stages.