Thin metal overlayers deposited on GaAs(110) cleavage surfaces enhance the surface oxidation rate several orders of magnitude above the clean surface value. The magnitude of this effect depends on the chemistry of the overlayer, on the local morphology of the overlayer–semiconductor interface region, and on the nature of the gaseous reactants. We present a synchrotron radiation photoemission investigation of the GaAs surface interaction with oxygen in the presence of Ag, Au, and Cr overlayers, and of the GaAs–H2O reaction in the presence of Cr overlayers. We find that the promotion effect is maximum in the presence of Cr overlayers, and that the nature of the surface reaction products changes if oxygen or water is employed. In particular, in the case of oxygen, Ga and As‐oxide phases are found with high atomic oxidation states present. If water is employed, only Ga oxide/hydroxyl phases are found at the surface, on top of a Cr–As subsurface layer that remains largely unaffected by oxidation.