The chemical nature of the Ni/Si, Ni/Ni2Si and Si/Ni2Si interfaces have been investigated using x‐ray photoelectron spectroscopy. Peak position, line shapes, and envelope intensities are used to probe the compositional structure of these systems. Two approaches have been employed: one approach examines the advancing planar silicide front by dynamically monitoring the i n s i t u formation of Ni2Si. This has the advantage of allowing examination of a realistic interface which is bounded on either side by an extended solid. The second approach follows the development of the Si/Ni interface using UHV depositions of thin layers of Ni on Si <100≳. 4He+backscattering is used to follow the progression of the thin film reaction and to provide quantitative information on atomic composition. These experiments demonstrate that the Ni/Ni2Si interface consists of a Ni‐rich silicide transitional phase while the Si/Ni2Si interface shows a transitional structure which is correspondingly Si‐rich. Intensity analysis indicates that these interfacial regions are at least 22 Å wide for α‐Si substrates and 9–14 Å wide for crystalline Si. The as‐deposited Ni/Si interface cannot be described as a unique single‐phase, but rather as a chemically graded transitional region showing a composition which varies from Si‐rich to Ni‐rich silicides.