The F₂ treatment of a hydrogen-terminated Si(100) surface in a vacuum chamber was chosen to generate a densely fluorinated silicon surface. It was confirmed by IR-attenuated total reflection and X-ray photoelectron spectroscopy measurements that the Si-H bonds on the surface were replaced with the Si-F bonds and that 1.0 monolayer of the Si-F bonds was formed after 1×10⁴ L exposure. Mass spectrometry measurements of the volatile reaction products indicated that the Si-F bonds were produced by the following reaction: {SiH(surface)+F₂(gas)→Si-F(surface)+HF(gas)}. The variation of thus prepared surface in water was also studied. The 1.0 monolayer of Si-F bonds was found to be replaced with the 0.7 monolayer of Si-H bonds and the 0.2-0.3 monolayer of Si-OH bonds after dipping into water for 1 min. The replacement of the Si-F bonds with the Si-H bonds was considered to be the back-bond-breaking reaction by water. Such a reaction between water and the surface Si-F was observed for the first time.