K-promoted Rh/USY (molar ratio: K/Rh = 3) catalyst was found to exhibit high performance in preferential oxidation of CO in rich hydrogen. Such high performance was maintained in the presence of steam and CO2. The CO oxidation activity of the K-Rh/USY catalyst was independent of the partial pressure of H-2, while the activity of the unpromoted Rh/USY catalyst was decreased significantly in hydrogen-rich stream. The effect of potassium addition on the catalyst structure was investigated and is discussed in terms of the differences in the catalytic performance. (C) 2003 Elsevier B.V. All rights reserved.