Chemical bonding and reactions at the Fe/Si interface have been studied as a function of Fe overlayer thickness and annealing temperature. The chemical properties (composition, electronic structure, and reactivity) were observed under ultrahigh vacuum conditions using x‐ray photoemission, ultraviolet photoemission (UPS), and Auger electronic spectroscopy. Both core line and valence‐band states have been measured. At room temperature, intermixing of atoms across the clean Fe/Si interface occurs. In the initial stage the Fe2p3/2core line shifts 0.4 eV to a higher binding energy relative to pure Fe metal. This is nearly the same as the chemical shift of the Fe2p3/2core line in FeSi2. With increasing coverage, the Fe2p3/2core line shifts decrease and subsequently approach that of pure Fe metal. In the meanwhile, the Fe2p3/2core line shapes changed gradually with greater width and asymmetry upon metal deposition. At the lower coverage, the UPS spectra are dominated by a broadband, but at high coverage, two peaks corresponding to Fe3dand Fe3d‐Si3pbonding are observed. Upon deposition,the nonbondingd‐state peak shifts to the Fermi level and the spectrum becomes that of pure Fe metal. All these results are discussed in the context of an interstitial diffusion model between Si and Fe at the Fe/Si interface.