Diffusion couples of aluminum nitride (AlN) and Ti were annealed under an argon atmosphere at temperatures ranging from 1300°to 1500°C for 0.5-36 h. The morphologies, crystal structures, and chemical compositions of the reaction zones at AlN/Ti interfaces were characterized using analytical scanning electron microscopy and analytical transmission electron microscopy. An interfacial reaction zone, consisting of TiN, τ 2-Ti 2AlN, τ 1-Ti 3AlN, α 2- Ti 3Al, and a two-phase (α 2-Ti 3Al+α- Ti) region in sequence, was observed in between AlN and Ti after annealing at 1300°C. The α 2-Ti 3Al region revealed equiaxed and elongated morphologies with and . In the two-phase (α 2- Ti 3Al+α-Ti) region, α 2-Ti 3Al and α-Ti were found to satisfy the following orientation relationship: and . The γ-TiAl and a lamellar two-phase (γ-TiAl+α 2- Ti 3Al) structure, instead of τ 1-Ti 3AlN, were found in between τ 2-Ti 2AlN and α 2-Ti 3Al after annealing at 1400°C. The orientation relationship of γ-TiAl and α 2-Ti 3Al in the lamellar structure was identified to be as follows: and . Compared with the reaction zone after annealing at 1400°C, the γ-TiAl was not found at the interface after annealing at 1500°C. The microstructural development resulting from isothermal diffusion at 1300°C and subsequent cooling at the interface are explained with the aid of the Ti-Al-N ternary phase diagram and a modified Ti-Al binary phase diagram.