The microstructural development of an AlN/Ti diffusion couple, annealed at 1000 degrees 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 delta-TiN layer was initially formed in the reaction zone between AlN and Ti, and the alpha(2)-Ti3Al layer subsequently developed between delta-TiN and Ti. Then an intergranular tau(1)-Ti3AlN phase was formed in the delta-TiN layer with the orientation relationships [111](tau 1)- Ti-3 AlN//[111](delta- TiN) and (1 (1) over bar0)(tau 1)-Ti-3 AlN//(1 (1) over bar0)(delta- TiN). The further diffusion of N atoms into the alpha(2)-Ti3Al layer led to the growth of delta-TiN and a twinned alpha(2)-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 tau(1)-Ti3AlN. The orientation relationships between tau(1)-Ti3AlN and alpha(2)-Ti3Al(N) were [111](tau 1)- Ti-3 AlN// [0001](alpha 2)- Ti3Al(N) and (0 (1) over bar1)(tau 1)- Ti3AlN//((1) over bar(1) over bar 20)(alpha 2)- Ti3Al(N). Finally, diffusion paths are proposed for the interfacial reactions at various stages.