An essential requirement for the proton exchange membrane (PEN) fuel cell is to deliver clean H-2 to the anode electrode, the kinetics of which are greatly hindered by traces of CO present from the upstream hydrocarbon steam reforming and water-gas shift processes. A technology receiving much attention is the selective oxidation of CO to CO2. Ideally the catalyst must selectively oxidize about 1% (10 000 ppm) CO to less than 5 ppm without oxidizing any of the 30-70% H-2 present. This paper describes performance results of a Pt promoted catalyst deposited on a monolith for selective oxidation of CO under simulated fuel processing conditions for PEM fuel cell applications. The effect of space velocity, gas composition and temperature on activity and selectivity is presented. Preliminary mechanism ideas are also presented.