Surface compositional modifications of binary alloys during 15‐keV O+2bombardment under 45° incidence were studied using Auger electron spectroscopy under ultrahigh vacuum conditions. For A1‐Ni, Cr‐Ni, Fe‐Ni, and Pd‐Ni alloys, the reactive preferential sputtering by O+2causes a surface enrichment of the alloy component which has the smaller metal‐oxygen bond energy per metal atom. For Cu‐Ni and Ag‐Pd alloys, where oxidation by the O+2beam was slight, the surface compositions after O+2bombardment were qualitatively similar to those caused by nonreactive preferential sputtering with Ar+.