The reactions of adsorbed CO on a carbon supported Pt catalyst (20% Pt/Vulcan) with O 2 in a 2% O 2 /Ar saturated electrolyte at potentials of 0.06 V and for comparison, at 0.26 V R H E are investigated by differential mass spectrometry under continuous flow conditions. Under these conditions, which are typical for an air bleed operation of polymer electrolyte fuel cell (PEFC) anodes, and on a CO saturated Pt/Vulcan, catalyst the reaction is found to be extremely slow, on the order of 10 - 5 monolayers s - 1 . Only a very small fraction, about 10 - 4 , of the consumed O 2 is used for CO oxidation, while the major fraction reacts via the competing oxygen reduction reaction, either in a four-electron reaction to H 2 O or, more likely, in a two-electron reaction to H 2 O 2 . CO stripping experiments confirm that at 0.06 V R H E O 2 reduction is possible even on a completely CO saturated Pt/Vulcan catalyst at a rate limited by O 2 transport. This is tentatively attributed to H 2 O 2 formation on the Vulcan support, which cannot be reduced to H 2 O on the CO covered Pt particles. At higher potentials, at 0.26 V R H E , the reaction rate is much higher, by a factor of about 100 for the CO saturated surface, but CO oxidation is still a minority reaction compared to O 2 reduction to H 2 O or H 2 O 2 . Here O 2 reduction can proceed only after partial CO a d removal, either by reaction with O 2 or by CO stripping. The completely CO covered catalyst is inert under these conditions. Consequences for the air bleed operation of PEFCs are discussed.