In preferential CO oxidation in H-2-rich stream (PROX), the additive effect of potassium on Pt catalysts was more remarkable over Al2O3 than over SiO2, ZrO2, Nb2O5 and TiO2. The additive effect of potassium to Pt/Al2O3 was more effective than other alkali metals. Especially, the presence of H-2 drastically promoted CO oxidation over Pt/Al2O3 modified with potassium. On the other hand, the suppressing effect of steam on the PROX was more significant over K-Pt/Al2O3 than over Pt/Al2O3, although the PROX activity of K-Pt/Al2O3 was much higher than that of Pt/Al2O3 even under the presence of steam. The different effect of steam addition to the PROX over K-Pt/Al2O3 Suggests that the active site can be Pt surface modified with potassium ions. (C) 2008 Elsevier B.V. All rights reserved.