A theoretical investigation of the Ni-Si(001) and Ni-Si(111) reactive interfaces using electronic band-structure calculation is presented. The following near-surface structures and interface models have been studied: (a) metastable adamantane structures of Ni interstitials in bulk Si at different stoichiometries: (b) isolated Ni interstitials near Si(001) and Si(111) surfaces; (c) single layer of NiSi2 rotated by 180°on Si(111); and (d) Ni atom chemisorbed on a single layer of NiSi2 on Si(111). The electronic properties of these structures and models are described and compared with the available experimental data. The low-Ni-coverage experimental results obtained for the (001) Si surface are interpreted in terms of an interfacial phase containing Ni interstitials in an adamantane geometry. For the (111) Si surface the experimental data provide evidence of both an interfacial phase containing Ni interstitials in an adamantane geometry and an epitaxial NiSi2-Si(111) interface.