The in situ surface oxidation of polycrystalline Ti-44Al-11Nb (compositions are in atomic %) alloy was studied at room temperature in an ultrahigh vacuum (66.6 to 80.0 nPa). The native oxide of Ti-44Al-11Nb was removed by sputtering the surface using 2.8 kV argon ions and then high-purity oxygen was carefully dosed onto the aluminide surface. After each oxygen dosing the surface species, metallic states and oxidized states, were detected by X-ray photoelectron spectroscopy (XPS). After analyzing the binding energy shifts and the concentration of oxidized species, aluminum oxidized first, then titanium followed by niobium. A reaction sequence model, incorporating the rate-determining step of a dissociative adsorption of oxygen, shows to be consistent with the initial oxidation rate of the Ti-44Al-11Nb intermetallic. (C) 2003 Kluwer Academic Publishers.