Initial oxidation processes of the Si(110) surface and the chemical bonding states of silicon atoms in the initial oxides have been investigated by using real-time synchrotron-radiation photoemission spectroscopy. Time evolutions of the Sin+ (n = 1-4) components in the Si 2p spectrum indicates that the Si3+ component always overwhelms the Si4+ component during the oxidation up to one monolayer. This is in sharp contrast to the Si(001) surface where Si4+ > Si3+ always holds. The dominance of the Si3+ component is related to presence of two types of bonds on the Si(110) surface and to their possible different reactivity against insertion of oxygen atoms. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3021032]