Carbon monoxide oxidation in a porous polymer electrolyte fuel cell (PEFC) electrode with PtRu/C catalyst was studied with steady-state polarization curves and open- circuit decay measurements. The Tafel slope was about 210 mV/decade and the reaction order for CO was about 0.45 at 0.4 V vs. RHE. This experimental behavior is explained with a mathematical model with CO adsorbing on Pt and water adsorbing on Ru. Kinetic parameters are determined from a fitting of the model to both the steady-state and the transient measurements. A single rate-determining step cannot account for the polarization curves over the whole potential range. At mid-range potentials the oxidation step is rate-determining but at lower potentials the water adsorption might be rate-determining. The open- circuit decay measurements gave noise-free measurements and confirmed the accuracy of the steady-state measurements. The obtained model for CO oxidation where the coverage of COads on Pt is determined can be used together with a model for H-2 oxidation. (C) 2004 The Electrochemical Society.