New synchrotron radiation results are reported for adsorption of oxygen at room temperature on Si(111)-7×7 surfaces in the 1-104 L exposure range. Core-level Si 2p spectra measured at 150 eV are confronted to O 1s spectra measured at 590 eV with high surface sensitivity and high energy resolution. The main finding is the presence, within the whole adsorption range, of two O 1s components separated by 1.5 eV with an intensity ratio of about 1:4. The main component at lower binding energy is defined with bridging oxygen atoms whereas the component with smaller intensity is related to surface atomic non-bridging oxygen atoms which are converted into bridging oxygen atoms upon heating. These results show that chemisorbed oxygen and oxide-like bridging oxygen coexist and that oxidation dominates at coverages as low as 0.2 monolayer. © 1986.