Reactions of surface Si-H and Si-Si bonds at H-terminated Si(100) and -(111) have been studied by measurements of FTIR and XPS spectra and flat-band potentials. The reactivity of the bonds in concentrated hydrogen halide (HX) solutions strongly depended on the presence or absence of an oxidant such as dissolved air, Br-2, and I-2 in the solution. With no oxidant, monohydride (=Si-H), dihydride (=SiH2), and trihydride (-SiH3) bonds on Si(100) and -(111) were stable, whereas, with an oxidant, they changed to SI-X (or Si-X-n,). In addition, back-bond oxidation occurred for Si(100) with an oxidant. All the reactions were explained hv hole injection by an oxidant, followed by nucleophilic attack of halide ions or water molecules. Interestingly, the rate of reaction of Si-H to Si-X for monohydride on (111) was higher than that for dihydride on (100), though the back-bond oxidation was negligible for(111) in contrast to the case of (100). The Si-X bonds on Si(lll) gradually underwent back-bond oxidation upon rinsing with pure water, finally recovering Si-H bonds through surface etching. It is discussed that charge polarization and steric hindrance in the surface bonds are two main factors in determining their reactivity.