The reaction of preadsorbed CO with O-2 and the simultaneous O-2 reduction reaction (ORR) on a carbon supported Pt catalyst (20%Pt/Vulcan) in an O-2-saturated electrolyte at potentials of 0.06 V and, for comparison, at 0.26 V-RHE were investigated by electrochemical measurements in a dual thin-layer flow cell under continuous flow conditions. Following a previous differential electrochemical mass spectrometry study (J. Electroanal. Chem. 2003, 554-555, 427), which demonstrated very Slow CO2 formation, in combination with significant O-2 consumption, even at the most cathodic potential (0.06 VRHE), we here show that the O-2 consumption oil the COad-saturated catalyst is due to almost exclusive H2O2 formation. Both the CO oxidation rate and the selectivity for H2O2, formation decrease with lower COad coverage, reaching slightly above 25% at the limiting coverage of 80% of the saturation value (0.6 monolayers). The experimental observations are explained by a mechanism, where H2O2 formation and, with a much lower probability, reaction with a neighboring COad occur upon O-2 adsorption in monomer vacancies. For O-2 adsorption on larger vacancy islands dissociative adsorption and subsequent H2O formation prevails, reaction with COad at the perimeter of the vacancy island is practically inhibited. Consequences for the air bleed operation of polymer electrolyte fuel cells are discussed.