[en] Energy loss spectroscopy in combination with AES and HEED is used to study and clarify the nature of intrinsic and oxygen-associated surface states. The c(2 x 8), (4 x 6), and (1 x 1) GaAs(100) surfaces obtained by in situ molecular beam evaporation exhibit different intrinsic surface states. The oxygen adsorption rates also differ, being largest for the Ga-stabilized (4 x 6) surface and smallest for the As-stabilized c(2 x 8) surface. Oxygen chemisorbs to form an unknown molecular complex common to all three surfaces. The oxidation behavior on Ge suggests two different adsorption sites, one saturating at approximately 50 percent of a monolayer. It is established that oxygen chemisorbs to form GeO molecules, which are weakly bound to the surface. 11 fig, 45 references