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-RHE 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 H2O or, more likely, in a two-electron reaction to H2O2. CO stripping experiments confirm that at 0.06 V-RHE 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 H2O2 formation on the Vulcan support, which cannot be reduced to H2O on the CO covered Pt particles. At higher potentials, at 0.26 V-RHE, 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 H2O or H2O2. Here O-2 reduction can proceed only after partial COad 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. (C) 2003 Elsevier B.V. All rights reserved.