The phase Ni2Si has been found to form on reacting evaporated Ni films with Si single-crystal substrates at temperatures ranging from 200° to 325°C. Its microstructure, impurity content, depth compositional profile and thickening kinetics have been studied by Seemann-Bohlin X-ray diffraction, nuclear backscattering and Auger electron spectroscopy with in situ ion sputtering. The thickening of the Ni2Si layer, which develops fiber-type texture with grains averaging 600 Å, was found to follow a parabolic relationship with time and to require an activation energy of 1.5±0.1 eV. During the growth of Ni2Si, the Ni has been shown by marker experiments to diffuse faster than the Si. Implanted Xe atoms were used as the diffusion marker, displacements of which were determined by backscattering. Combined measurement of layer thickness and marker displacement showed that the mass transport in Ni2Si was almost completely dominated by Ni atoms. If we assume diffusion occurs by exchange with lattice vacancies, diffusion of the Ni is favored from a consideration of the first nearest neighbor configurations in the crystal structure of Ni2Si and the Goldschmidt atomic radii of Ni and Si. The existence of excess vacancies in the Ni sublattice is not certain from the results of depth compositional profiling; it has shown, however, a pile-up of Si at the Ni free surface. The pile-up may be due to the short-circuit diffusion of Si atoms through the Ni and the Ni2Si layer.