Polymer electrolyte fuel cells operate at high efficiencies using pure Hz fuel. Hz produced by reforming hydrocarbons or alcohols contains CO and CO2 (COx) impurities which readily adsorb onto anode Pt electrocatalysts, reducing the efficiency for H-2 electrooxidation. A COx inventory model is useful for describing the behavior of adsorbed CO on Pt electrocatalyst surfaces. The model compares three fluxes : (1) direct adsorption of CO, (2) the electroreduction of CO2, and (3) electrooxidation of adsorbed CO. The fluxes of CO2 electroreduction and CO electrooxidation were measured electrochemically on polycrystalline Pt electrodes (surface roughness = 30-100) in 1 N H2SO4 between 23 and 66 degrees C. This study concludes that, under polymer electrolyte fuel cell conditions, CO tolerance is achieved when the flux of CO electrooxidation balances the combined adsorption fluxes from both CO and CO2. Furthermore, under most conditions, CO adsorption will generally exceed the fluxes of CO2 electroreduction.