The addition of potassium enhanced the activity of preferential CO oxidation on Pt/Al2O3. The additive effect of potassium weakened the interaction between CO and Pt, and also changed the CO adsorption site. FTIR observations under the PROX conditions suggested the presence of adsorbed species derived from O-2 and H-2 such as OH species, which can be related to the promoting effect of H-2 present in the PROX reaction. (c) 2007 Elsevier Inc. All rights reserved.