X-ray photoemission spectroscopy (XPS) was used to investigate the chemical reactivity and band-bending variation of native oxide covered surfaces of GaAs (100) during Schottky-barrier formation. The GaAs surfaces prior to metal deposition had thin ∠10 Å overlayers of either As2O3 and Ga2O3 or only Ga2O3. A variety of metals, some chemically inert (Au, Cu, and Ag) and some chemically reactive (Al, Mg, Cr, and Ti), were studied on both types of oxide surfaces. The chemical reactions occurred at room temperature and were well predicted by bulk thermodynamic free energies of formation.