Interfacial reactions between Si and deposited metals have been investigated by x‐ray photoelectron spectroscopy (XPS) and channeling measurements with MeV 4He ions. This paper is concerned with reactions of Ni deposited at 10−10 Torr on clean Si and with the structure of the interface in the Si/Ni2Si/Ni structure. Both XPS and channeling measurements show that reactions occur where Ni is deposited at 10−10 Torr on Si: Si atoms are displaced from lattice sites, the Ni atoms are in an Si‐rich environment, and the Ni/Si interface is graded in composition. Composition gradients are present at both interfaces in the Si/Ni2/Si/Ni system: at the Si/Ni2Si interface Ni atoms are in a more Si‐rich environment than that found for Ni in Ni2Si or NiSi and at the Ni/Ni2Si interface Si atoms are in more Ni‐rich environment than that found for Si in Ni2Si. In both deposited Ni on Si and the Si/Ni2Si/Ni structures, Ni atoms are found at the tetrahedral interstitial site in Si. For the Ni–Si system, cooling the substrate to ∠100 K slows down the reaction rate. The temperature dependence of the interfacial reactivity indicates the kinetic nature of metal–semiconductor interfaces.