The addition of alkali metals over Pt/Al2O3 has both promoting and negative effects on the catalytic performance in the preferential CO oxidation in H-2-rich stream (PROX), therefore there is an optimum amount of alkali metal. The Pt/Al2O3 catalysts modified with Na, K, Rb and Cs were characterized by means of transmission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS), Xray absorption near-edge structure (XANES), and Fourier transform infrared spectroscopy (FTIR). The results show that the addition of larger amount of alkali metals with stronger basicity causes the aggregation of Pt metal particles. The Pt particles on Pt/Al2O3 modified with alkali metals are more electron-deficient than those on Pt/Al2O3, and this weakens the strength of CO adsorption on Pt/Al2O3 modified with alkali metals, which is related to the enhancement of turnover frequency of the PROX. In addition, in situ FTIR observation suggests that the coadsorbed species originating from H-2 and O-2 (e.g., the OH species) under the PROX condition, which promote the CO oxidation, can be found on highly active catalysts. (C) 2008 Elsevier B.V. All rights reserved.