The formation of sacrificial oxides on GaAs is a well established procedure in the preparation of in situ cleaned substrates for epitaxial growth. Oxides formed from ozone exposure promise a more controlled alternative to thermal and air formed oxides. This paper reports on a study of the composition and structures of oxides on GaAs(100) surfaces and, in addition, examines the persistence of carbon-containing species at the oxide surface and oxide–semiconductor interface. X-ray photoelectron spectroscopy (XPS) was used in a high spectral resolution mode, to distinguish between the different oxide phases formed. Surfaces oxidized under ozone for 10–30 min had Ga/As oxide ratios close to unity. By contrast, acid treated, DI water rinsed surfaces exposed to air produce oxides rich in either Ga or As depending on conditions. Heating of the ozone-produced oxide to ≥600 °C results in sequential decomposition of As2O5, As2O3, and Ga2O3. Oxide layers produced by short-term ozone exposures were analyzed by angle-resolved XPS. A 10 min exposure produced an oxide of 2.0 nm thickness. Surface carbon-containing contaminants were removed to a level ≤0.2 monolayer by heating the specimen to 400 °C in vacuum (base pressures of 2×10−8 Pa). Surfaces heated to ≥600 °C to desorb all oxides, also showed no evidence of carbon-containing species and the As/Ga ratio was unity.