The preferential CO oxidation on a monolith Ru/Al2O3 catalyst pretreated with SO2 in the reactant gas was investigated in the presence/absence of 0.21-20 ppm sulfur compounds (SO2 or H2S). In the presence of 2.1 ppm sulfur compound, CO was preferentially oxidized over the SO2-pretreated catalyst at 150 degrees C. On the other hand, in the absence of sulfur compounds, the H-2 oxidation was promoted. By in situ IR measurements, multicarbonyl species and SO42- species were found on Ru in the presence of SO2. The presence of multicarbonyl species at a high frequency (2154 cm(-1)) and that of SO42- species on Ru suggested that Ru was oxidized. Since the rate of CO oxidation was low on oxidized Ru, the selectivity for CO oxidation was low over the SO2-pretreated catalyst in the absence of sulfur compounds. On the other hand, the increase in the selectivity in the presence of sulfur compounds suggested that weakly adsorbed sulfur compounds suppressed the migration of dissociatively adsorbed H atom on Ru or decreased the H atoms next to adsorbed O atom for the H-2 oxidation. (c) 2006 Elsevier B.V. All rights reserved.