Density functional theory is used to calculate surface segregation tendencies of iridium atoms in Ir (core)-Pt (shell) systems with various shell thicknesses, on clean surfaces and under 1/4 monolayer of atomic oxygen adsorbed on fcc and hcp sites of (111) surfaces. Iridium shows strong antisegregation behavior in clean surfaces, but the behavior reverts dramatically in the presence of adsorbed oxygen. On Pt-skin surfaces, where the oxygen adsorption is the weakest, monolayers have better protective effect against segregation of the less noble metal, and the effect decreases as the number of layers in the shell increases. (C) 2008 Elsevier B.V. All rights reserved.