An Al oxide‐GaAs (100) interface fabricated by the reactive deposition of Al into a molecular oxygen overlayer on a gallium terminated GaAs (100) surface atT=49 K is studied by synchrotron radiation photoemission. Al forms a stable oxide layer by reaction with O2until all the oxygen is consumed. Limited oxidation of surface As atoms (≊20%) is observed during the initial deposition of Al, but further Al deposition reduces the AsO bond. The well‐known exchange reaction between Al and Ga when Al is directly deposited on GaAs (100) is not observed.