The initial stages of the reaction between thin Al grains with an average diameter of 2 × 10−5mand sputter deposited, nanocrystalline Cu films has been studied in plan view using in situ transmission electron microscopy. At high reaction rates, the phase transformation in the Al grain resulting from the interdiffusion process is found to exhibit metastable growth morphologies characterized by negative curvature at the growth interface. The crystal structure of the initial phase formed in the Al grain under relatively high reaction rates is a metastable, orthorhombic distortion of the equilibrium body centered tetragonal θ phase. The distortion is found to vary with Al grain surface orientation. The degree of metastability can be experimentally correlated with the kinetics of the interdiffusion process as controlled by diffusion barrier thickness.