The interaction of unexcited oxygen with vacuum‐cleaved GaAs(110) surfaces has been studied by XPS (hν=1253 eV) and UPS (hν=21.2, 40.8, and 48.4 eV) over the exposure range 105to 1014L. The corresponding coverage range is estimated as ϑ∠0.2 to ϑ∠0.8 monolayers (where ϑ=1 corresponds to an oxygen atom for each surface atom). From the UPS and O 1sXPS results it is concluded that the adsorption is dissociative over this entire range. From a study of the Ga and As 3dcore levels; and the Ga and AsL3MMx‐ray‐induced Auger lines it is shown that chemical shifts associated withbothAs and Ga levels occur over the entire adsorption range. These shifts are appropriate to those for Ga2O3and As2O3, as is the stoichiometry of the reacted layer. There is no direct evidence for a stable chemisorption state preceeding oxidation in the 105to 1015L exposure regime. It is suggested that reaction with molecular oxygen takes place by dissociation at defect sites leading to growth of Ga and As oxides.