CO oxidation in the absence or presence of hydrogen has been investigated in the 0.4-15 kPa range over Ir(1 1 1), Ir(1 1 2), Pt(1 1 1) and Pt(1 1 2) single-crystal surfaces. Iridium appears more active than platinum in CO oxidation, and more active and selective than platinum in preferential CO oxidation in the presence of hydrogen (PROX), both under stoichiometric and oxygen-rich CO:O-2 conditions. Stepped surfaces are more active than flat surfaces. These results are not consistent with a CO-blocking model if only adsorption energies of pure CO are taken into account. A beneficial effect of a small amount of hydrogen on CO oxidation over Pt and Ir surfaces has been evidenced under specific conditions. Experiments on Pt(1 1 1) show that it is not related to water formation and does not originate from an irreversible H-2-induced surface restructuring. (C) 2012 Elsevier B. V. All rights reserved.