Photoemission and work function measurements are used to investigate the formation and structure of Ni–Si(100) interfaces at 300 K. For Ni coverages θ≲0.5 Å (4.6×1014/cm2) a chemisorbed phase of Ni surface atoms forms. This chemisorbed phase persists to coverages θ≂2 Å; but also in this coverage range, a diffusion layer forms in the Si lattice. At θ≊1.5 Å the surface composition closely resembles NiSi. Addition of more Ni atoms (θ≥1.5 Å) initiates nucleation of Ni2Si. The growth of this phase continues up to θ≊1.5 Å where silicide formation stops. Additional Ni atoms deposit as a pure Ni overlayer. These results yield a model for Ni–Si interfaces formed at room temperature which consists of a shallow diffusion layer of Ni atoms in the Si lattice, a very thin (1.5-Å thick) interface having NiSi chemical characteristics, a Ni2Si phase (15-Å thick) of relatively uniform stoichiometry followed by a Ni layer.