We have studied the thermodynamics and kinetics of growth of Al-Ti phases in thin-film interdiffusion couples, A lOOnm thick aluminum layer is vapor-deposited on a poly- or single-crystal titanium substrate, previously etched by ion bombardment. The sample is then annealed during a time ranging from a few minutes to a few hours, at temperatures from 300 to 600°C. The Al atomic fraction vs depth is determined using a sputtering technique, associated with thermo-ionization mass spectrometry (STIMS). At low temperature (300–450°C), titanium diffuses along the grain boundaries of the micro-crystalline aluminum layer, to form an Al-rich phase. At higher temperature (450–500°C), the T1AI3 phase grows rapidly up to the surface. Above 460°C, an intermediate Ti-72at/Al phase grows between TiAl3 and Ti‥ At 600°C, the aluminum layer becomes morphologically unstable.