Interdiffusion and reaction at the interface between titanium thin films and AlN have been studied by using Rutherford backscattering spectrometry and transmission electron microscopy. Ti 2 AlN is formed as a result of reaction with titanium and AlN at temperatures of 800°-950°C. The activation energy for Ti 2 AlN formation in the temperature range of 800°-850°C is 224 kJ/mol, which is similar to that of nitrogen diffusion in titanium. Therefore, the formation of Ti 2 AlN is believed to be controlled by the diffusion of nitrogen in titanium.