Initial oxidation processes and local bonding structures of hydrogen-terminated Si(100)-2×1 and H₂O-terminated Si(100)-2×1 surfaces have been examined by high-resolution energy loss spectroscopy (HREELS). The hydrogen adsorption on Si(100) surfaces suppress the oxidation of dimer bond sites, and oxygen atoms preferentially adsorb on one of the two back-bond sites of surface Si atoms. On the other hand, oxidation proceeds randomly up to an oxygen coverage of 3 ML on H-terminated Si(111) surfaces. The Si-O-Si bonds formed on H-terminated Si(100) surfaces are more relaxed than those on clean Si(100) surfaces, which is considered to originate from the change in bond angles of Si-O-Si bonds. The H₂O-terminated Si(100)-2×1 surfaces are also stable for the adsorption of oxygen molecules. However, the uptake of oxygen atoms of Si-OH species into back-bond sites occurs even at room temperature by the reaction of H₂O-terminated Si(100) surfaces with atomic hydrogen.