The oxygen adsorption processes during the initial oxidation of Si(100) have been investigated by high-resolution photoemission spectroscopy of the Si2p core levels. The variation of the Si2p intensities was measured in detail for the different sub-oxide (Si 1+ , Si 2+ , Si 3+ , and Si 4+ ) components. The oxygen adsorption processes at room temperature during the initial oxidation were quantitatively analyzed by measuring the intensity ratios of each sub-oxide component as a function of the oxidation time. It is found that the Si 1+ and Si 2+ species are localized mostly at the first interfacial layer, while the Si 3+ and Si 4+ components exist in the two-dimensional islands with certain height on the initial oxidation layer, and are expanded horizontally during further oxidation. Our results suggest that the two-dimensional island nucleation occurs during the initial oxidation.