Auger electron spectroscopy is used to follow the initial chemisorption of oxygen on a Si(100)−2 × 1 surface. Careful measurements of the Si L23VV spectra reveal, for the first time, fine structure in the region between 80 and 85 eV. For the clean Si(100) surface a peak at 81 eV has been found. Oxygen chemisorption on the Si(100) surface is associated with the appearance of a new well defined peak at 83 eV in addition to the peak at 81 eV. Since these features are discernible for the (100) crystal face, the 83 eV transition is assigned as due to the Si---O bond.