In situ ir studies of semiconducting-electrode surfaces may be advantageously performed by using internal-reflection geometry with multiple reflections. Here the surface of silicon has been investigated at the interface with organic electrolytes. HF-rinsed silicon exhibits a surface covered with covalently bonded hydrogen. The (111) and (100) crystal surfaces exhibit distinct features (proportion of SIH, SiH2 and SiH3 groups). Use of the polarization of the infrared beam brings further information about the geometry of these groups. The nu SiH vibration frequency and linewidth are found to be affected by the interactions with the electrolyte. When the surface gets oxidized due to residual water in the electrolyte, oxidation of the SiH groups at the (111) surface involves fast insertion of oxygen atoms into all three silicon backbonds. On the other hand, at the (100) surface, oxidation is more progressive, as indicated by the existence of SiH groups backbonded to 1, 2 or 3 oxygen atoms.