We have used photoemission techniques with synchrotron radiation to study the adsorption of oxygen on the Si(111)-(7×7) and Si(100)-(2×1) surfaces at room temperature in the submonolayer and monolayer regimes. High resolution Si 2p core-level spectra, valence-band spectra, work-function, and Fermi-level pinning positions have been measured for exposures between 1 and 1000 L and after annealing at 700 °C. Four oxidation states have been detected for the silicon surface atoms with Si 2p core-level shifts of 0.9, 1.9, 2.6, and 3.4 eV which are assigned to silicon atoms bonded to 1, 2, 3, and 4 oxygen atoms. Both (111) and (100) surfaces are characterized by about the same sticking coefficient, a simple adsorption process for 1-L exposures, the quenching of the surface states after 15 L and a saturation of the amount of oxygen after 100 L. Significant differences exist in the initial bonding geometries for the (111) and (100) surfaces. After 15-L exposure, oxygen atoms are incorporated below the Si (111)-(7×7) surface to form SiO4 units.