A standard cleaning procedure for GaAs (100) molecular beam epitaxial (MBE) substrates is a chemical treatment with a solution of H2SO4/H2O2/H2O, followed byinsituheating prior to MBE growth. X‐ray photoelectron spectroscopic (XPS) studies of the surface following the chemical treatment show that the oxidized As is primarily As+5. Upon heating to low temperatures (<350 °C) the As+5oxidizes the substrate to form Ga2O3and elemental As (As0), and the As+5is reduced to As+3in the process. At higher temperatures (500 °C), the As+3and As0desorb, while the Ga+3begins desorbing at ∼600 °C.