We have investigated the mechanism of Hf silicidation from Hf O2 gate insulators on Si by high-resolution core-level photoemission spectroscopy with detailed annealing-temperature controlling. We have found that the silicidation temperature depends on the difference of the chemical states at the interfacial layer. The Hf-silicate layer which is more stable than the Si O2 layer prevents the silicidation. In addition, silicidation processes also promote the formation of Si O2. Chemical shifts in core-level photoemission spectra depend on the interfacial-layer thickness and Si O2 concentration in the Hf O2 top layer, which are tunable by detailed annealing temperature.