The effect of H2S (22 ppm) on the reaction of CH4 With CO2 over Rh/TiO2 and Rh/SiO2 catalysts was examined. For comparison the reaction was studied on Al2O3, ZrO2 and CeO2-supported Rh, too. It was found that H2S suppresses the formation of CO and H-2 in all cases, but on Rh/TiO2 and on Rh/ZrO2 the CO/H-2 ratio increased in time, indicating that H2S poisons the secondary processes of the reaction in different extent. In the presence of H2S the amount of surface carbon formed in the reaction decreased significantly but its reactivity remained the same. The effect of H2S on the CH4 + H2O and on the CO2 + H-2 reactions was also studied. The rate of methane formation was unexpectedly higher in the CO2 + H-2 reaction on Rh/TiO2 in the presence of H2S. TPD measurements showed that H2S desorbed as SO2 from Rh/TiO2. The mechanism of this promoting effect is proposed. (C) 2004 Elsevier B.V. All rights reserved.