The interaction of oxygen with a Ni 76%/Fe 24% (100) surface has been studied at 293 K. XPS measurements were made as a function of emission angle to obtain depth distributions. Slight Fe segregation is observed for a clean annealed surface. It is enhanced by oxygen adsorption. By 0.7 monolayers oxidized Fe species are detected, but the Ni remains metallic until well above one monolayer. The Fe concentration in the outer 4 Å has then increased twofold. At about ∠2.7 monolayers (passivation) the surface Fe concentration is nearly three times the original and entirely oxidized. Heating causes further Fe segregation which reduces the Ni2+ to yield Ni0. LEED shows a c (2×2) pattern which persists above one monolayer. Correlations of the XPS and LEED data and comparison with XPS/LEED studies of oxygen on Fe(100) and Ni(100) show that between 0.7–2 monolayers, the surface must be heavily patched with Fe oxide and metallic Ni aggregations, the Ni being covered with a c (2×2) oxygen superstructure.