The coverage dependent interaction of molecular oxygen with the Si (111) 7 × 7 surface at 20 K has been studied with high resolution electron energy loss spectroscopy, ultraviolet photoelectron spectroscopy, and low energy electron diffraction. These results provide a new view of the initial stages of oxidation of silicon. In addition to physisorbed molecular oxygen, two other oxide species occur at monolayer coverages: diatomic-like and bulk-like forms of silicon monoxide. Formation of the diatomic-like monoxide begins at the lowest coverages while the appearance of the bulk-like monoxide is delayed. The diatomic silicon monoxide is stable to a temperature between 475 and 575 K, and the bulk-like monoxide is stable to approximately 950 K. This latter oxide completely desorbs at approximately 975 K. The sticking coefficient for decomposition of molecular oxygen was found to be much greater at 20 K than at room temperature. This effect can be rationalized if it is assumed that a mobile precursor state, differing from those of metal surfaces, has a major role in the oxidation reaction. We also present evidence that the detailed vibrational spectra frequently observed for oxygen absorbed on Si (111) is complicated by the presence of a very small amount of hydroxyl contamination. © 1985.