The activity of carbon-supported Pt/MoOx (Pt/MoOx/C) toward oxidation of 100 ppm CO in H-2 was investigated using a rotating disk electrode in 0.5 M H2SO4 solution and by anodic polarization behavior in a single cell configuration. CO-stripping voltammetry of the Pt/MoOx/C catalyst showed a higher stripping peak potential compared to PtRu/C; however, the onset potential of the potentiodynamic oxidation of the CO/H-2 mixture was somewhat smaller than that of PtRu/C. Anodic polarization of a Pt/MoOx/C anode in a CO/H-2 mixture decreased with decreasing fuel flow rate and with decreasing thickness of the Nafion membrane, which indicated that part of the improved CO tolerance of the Pt/MoOx/C anode was due to O-2 that permeated from the cathode. This is supported by the anodic polarization of Pt/MoOx/C increasing and becoming independent of the fuel flow rate when the cathode gas was changed from O-2 to H-2. Analyses of CO concentration in the exhaust gas from the anode side showed that part of the CO in the fuel gas stream was oxidized by electrochemical and nonelectrochemical processes. The CO tolerance of the Pt/MoOx/C anode was attributed to the intrinsic CO tolerance of the catalyst and nonelectrochemical CO oxidation by the permeated O-2. (C) 2003 The Electrochemical Society.