The formation of ultrathin epitaxial layers of CoSi2 on Si(100) surfaces is studied by means of valence-band and core-level photoemission spectroscopy with synchrotron radiation, low-energy electron diffraction (LEED) and Auger electron spectroscopy. The CoSi2 films are prepared by the template method in which an amorphous layer with CoSi2 stoichiometry is coevaporated on top of a 2.6–3 monolayer thick Co overlayer at room temperature. The amorphous layer does not react to CoSi2. Annealing to 300°C leads to the crystallization of the layer demonstrated by the formation of a (✓2×✓2)R45° LEED pattern. This layer consists of disilicide in CaF2 structure with some contribution from another silicide phase. This surface is stable at least up to 460°C. After further annealing above 460°C the disilicide disappears, at least from the sub-surface layer, and the surface is covered by clean Si patches and patches of the second silicide phase. The new silicide phase is characterized by a LEED pattern and has a different electronic structure and Si 2p binding-energy shifts. A comparison with previous experimental results and existing theoretical electronic structure calculations suggests that the new silicide phase can be identified as adamantane disilicide.