Time-resolved in situ X-ray tomography combined with scanning electron microscopy was performed on an Al-Fe diffusion system at 973 K (700 A degrees C) to study the formation of the main intermetallic compounds occurring at the interface. After nucleation on the liquid side of the interface, growth occurs in both liquid and solid directions. In the direction of the solid, growth starts with a particular tongue-like feature which then progressively thickens. The thickening is linked to the deformation of the iron matrix during the formation of the intermetallic compound. Growth in the direction of the liquid is slowed down by erosion. (C) The Minerals, Metals & Materials Society and ASM International 2013