Transmission electron microscopy (TEM) has been utilized to study the nickel-silicide growth in self-supported lateral-diffusion, thin-film couples by overlapping deposited layers of Ni and Si between two silicon oxide deposited films. Energy-dispersive x-ray spectroscopy, microdiffraction, and selected area diffraction were used to identify the Ni-silicide phases and their crystal structures. Long-grain growth of Ni2Si, as a result of phase-boundary migration induced by diffusion, was observed during in situ annealing between 500 and 750 °C in TEM. No preferred orientation or particular crystallographic relationship was found among the long grains.