The adsorption of nonexcited molecular oxygen on cleaved GaAs(110) surfaces at room temperature has been studied using photoemission techniques. Detailed analysis of the oxygen-induced structure in the valence-band region revealed two different forms of adsorbed oxygen. Adsorption in the first form saturates at a very low coverage (∠0.01 monolayer) and is probably associated with defect sites. Adsorption in the second form occurs at normal surface sites and produces measurable chemical shifts in Ga-3d and As-3d core levels. The nature of the second form of oxygen has been further investigated with core level studies of surfaces oxidized at room temperature and subsequently heated to a high temperature. Annealing to moderately high temperature (∠370°C) causes transfer of oxygen from As–O bonds to form additional Ga–O bonds. Fast heating to high temperature (430°–450°C) leads to desorption of roughly half of the oxygen atoms and all of the chemically affected As atoms, while little change in the Ga-3d core level is observed. Several previously proposed oxidation models are compared based on the present data.