The influence of the addition of 2.1 or 21 ppm SO2 or 2.1, 4.3 or 48 ppm H2S to reaction gas composed of 51.5 vol.% H2O 0.3 vol.% CO2 13.0 vol CO2, 0.5 vol.% O-2, 6.0 vol.% N-2, and 28.8 vol.% H2O on the activity for the CO preferential oxidation was investigated. The catalysts used for the reaction tests were monolith Ru/Al2O3 and Pt/Al2O3 catalysts with Ru loading of 1.6 g/l and Pt loading of 3.1 g/l, respectively. Pellet 3.8 wt.% Ru/Al2O3 and 3.8 wt.% Pt/Al2O3 catalysts were also used to elucidate the deactivation mechanisms. The addition of a high concentration of SO2 or H2S decreased the activit for the CO preferential oxidation. The XPS and IR spectrum for the SO2-poisoned Ru/Al2O3 catalyst showed that Ru particles adsorbed SO42-species, while the XPS for both SO2- and H2S-poisoned Pt/Al2O3 catalysts showed that Pt particles adsorbed S2- species. The activities of the Ru/Al2O3 catalyst for both CO and H-2 oxidations were lowered in the presence of the sulfur compounds: the selectivity of CO oxidation was almost constant during the poisoning reaction. On the other hand, the activity of the Pt/Al2O3 catalyst for CO oxidation was lowered by small amounts ofthe sulfur compounds, while the activity for H-2 oxidation was almost constant: the selectivity of CO oxidation decreased. The poisoned Pt catalysts were regenerated by the calcination at 175 degrees C, and the covering S2- species was oxidized to SO42- species, which migrated to the Al2O3 support. (c) 2005 Elsevier B.V. All rights reserved.