In this work we have investigated in situ the nanoscale oxide formation on Cu{1 0 0}, Ag/Cu{1 0 0} surface alloy and polycrystalline Cu(Ag) alloy. The experiments were conducted in pure oxygen atmosphere (∼2 mbar) as a function of surface temperature (320–473 K). The concentration of metallic Cu, Cu2O and CuO in the surface region was quantified by X-ray photoelectron spectroscopy (XPS). Oxygen induced segregation of the alloy constituents in the near-surface region, as well as the diffusion of oxygen towards the bulk, were investigated using depth-profiles obtained by employing Ar+ ion sputtering and Auger electron spectroscopy (AES). We observed only Cu2O formation on all three surfaces under the oxidation conditions used in these experiments. The formation of Cu2O was found to be strikingly similar on both Ag/Cu{1 0 0} and Cu{1 0 0}. Our depth-profiling experiments indicate that this is due to a rapid oxygen induced segregation of Cu to the surface and subsequent formation of Cu oxides on top of the Ag overlayer. Evidence of a similar oxidation mechanism was also found on Cu(Ag).