The crystallographic, electronic, and optical properties of thin ReSi2films (∼20–300 A˚) have been investigatedinsituby low energy electron diffraction (LEED) and photoelectron spectroscopy (XPS and UPS),exsituby glancing incidence x‐ray diffraction (GIXD), and optical absorption measurements. Thin Re layers were evaporated under ultrahigh vacuum on Si(111) (7×7) surfaces, maintained at room temperature, or heated at 650 °C. The films were subsequently annealed at increasing high temperature and the silicide formation was followed byinsitusurface techniques. For very thin films (≲35 A˚) LEED shows a faint (1×1) pattern after annealing at 750 °C, which improves slightly up to ∼900 °C. For thick films (∼50–300 A˚) only a bright background is observed. XPS indicates that the ReSi2composition is attained upon annealing at 600 °C. In the Re‐Si bonding the charge transfer is negligible: the energy positions of the corelevels (Si 2pand Re 4f) are the same in the compound and in the elements. As the energy shift of the SiKLLAuger is negligible also, the extra‐atomic relaxation energy for Si atoms is the same in silicide as in silicon, indicating that ReSi2has a semiconducting character. UPS results confirm this assumption: the density of states nearEFdecreases strongly upon ReSi2formation and at the same time the valence band edge shifts fromEFto lower binding energy. GIXD gives sharp diffraction peaks, characteristic of ReSi2(110) in epitaxy on Si(111). This technique also reveals that the films present an additional orientation near the interface. Optical absorption measurements performed on ReSi2films of ∼300 A˚ thick indicate that this silicide is a semiconductor with an indirect energy gap of 0.15 eV, in agreement with previous studies. ©1995 American Institute of Physics.