Although oxidation of silicon has been intensively investigated experimentally and theoretically, the composition and structure of the interface region between the bulk oxide and silicon are not very well known. In this work, we report on the vacuum UV laser-induced oxidation of well-defined atomically flat Si(111)-(1 X 1):H and Si(110)-(1 X 1):H surfaces, prepared by wet-chemical processing, using an F-2 laser (157 nm). The silicon samples were irradiated under ultra high vacuum conditions in water and oxygen atmospheres. The photo-induced partial oxidation of the first monolayer, controlled by the oxidant pressure, was monitored by in situ Fourier transform infrared transmission spectroscopy with different photon detectors. The chemical reactions occurring at the surface were monitored by the shift and decreasing strength of the stretching vibration of Si-H surface bonds. The IR absorption spectra recorded during surface processing yield detailed information on the rearrangement of chemical bonds with a sensitivity reaching monolayer resolution.