The effects of magnesium on platinum catalyst used for the preferential oxidation of carbon monoxide for polymer electrolyte membrane fuel cell applications are investigated. The CO conversion and selectivity on Pt-Mg/Al2O3 for a H-2-rich stream are 93.1 and 62.0%, respectively, but only 70.2 and 46.89% on Pt/Al2O3. The superior activity of Pt-Mg/Al2O3 for the preferential oxidation of CO is due to an increase in the hydroxyl groups that results from an increase in basicity with the addition of Mg, as well as to an increase in the electron density on the surface of the Pt catalyst. Moreover, the content of hydroxyl groups on the Pt catalysts is promoted by water vapour. (c) 2005 Elsevier B.V. All rights reserved.