The effect of the Cs addition on the H2S inhibition and subsequently on the DBT HDS activity over Ru-based catalysts was investigated. In good agreement with a previous study, the HDS activity over a Ru catalyst containing cesium was much higher than that over the same Ru catalyst without cesium. On the other hand, the HDS activity of the Ru-Cs catalyst was more inhibited by H2S than that of the Ru catalyst at 260 degreesC. Thus, kinetic parameters were calculated using a Langmuir-Hinshelwood model to determine the mechanism of the H2S inhibition over Ru and Ru-Cs catalysts. We found that the heats of adsorption of DBT and H2S on the Ru-Cs catalyst were higher than on the Ru catalyst, indicating that the sulfur-containing species adsorb more strongly on the catalyst containing cesium. Subsequently, the role of cesium in the Ru-Cs catalyst on the DBT HDS activity was investigated by using S-35-tracer experiments. The results suggested that while the Ru-Cs catalyst was more inhibited by H2S, the Ru-S bonds were more stable on this catalyst than on the Ru catalyst. This Ru-S bond stabilization was responsible for the stabilization of the active phase, which allowed creation of a greater amount of sulfur atoms potentially labile. Thus, that explained the better HDS activity over the Ru-Cs catalyst than over the Ru catalyst, despite a greater H2S inhibition on the former. (C) 2004 Elsevier Inc. All rights reserved.