The chemical reactivity of Si-H (and SiH2) bonds at Ii-terminated Si(lll) surfaces immersed in hydrogen halide (HX; X = Cl, Br, and I) solutions has been investigated by measurements of FTIR and XPS spectra and Aat-band potentials. The decreases in the intensity of FTIR bands, together with the increases in the surface atomic (X/Si) ratios obtained from XPS spectra, clearly showed that Si-H bonds in the HX solutions changed to Si-X in the presence of oxidants such as dissolved oxygen and I-2. The conclusion was supported by large positive shifts in the flat-band potential of Si(111). The extent of the changes depended on the concentration and oxidizing power of oxidant and the immersion time. On the other hand, the Si-H (and SiH2) bands were fairly stable in the HX solutions, which contain no oxidant, showing only slight decreases in amount, less than about 6% after immersion for 10 min. It is discussed that the formation of Si-X bonds occurs through hole injection by an oxidant, followed by nucleophillic attack of halide ions. Spectral changes observed for the Si-H vibration bands are also discussed on the basis of the Formation of Si-X bonds.