Oxidation of Cu3Au(1 1 0) using a hyperthermal O-2 molecular beam (HOMB) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident energy dependence of the O-uptake curve, the precursor-mediated dissociative adsorption occurs, where the trapped O-2 molecule can migrate and dissociate at the lower activation-barrier sites, dominantly at thermal O-2 exposures. Dissociative adsorption of O-2 on Cu3Au(1 1 0) is as effective at the thermal O-2 exposure as on Cu(1 1 0). On the other hand, at the incident energies of HOMB where the direct dissociative adsorption is dominant, it was determined that the dissociative adsorption of O-2 implies a higher activation barrier and therefore less reactivity due to the Au alloying in comparison with the HOMB oxidation of Cu(1 1 0). The dissociative adsorption progresses with the Cu segregation on Cu3Au(1 1 0) similarly as on Cu3Au(1 0 0). The growth of Cu2O for 2 eV HOMB suggests that the diffusion of Cu atoms also contribute to the oxidation process through the open face, which makes the difference from Cu3Au(1 0 0). (C) 2010 Elsevier B.V. All rights reserved.