Alumina-supported ruthenium-cesium catalysts were presulfided using [S-35]H2S pulse tracer method to evaluate their sulfidation state. Subsequently, using these previously S-35-labeled catalysts, HDS reactions of dibenzothiophene (DBT) were performed and the mobility of S-35 introduced during the presulfidation stage was investigated. The results showed that the amount of labile sulfur (SO) was much smaller than the total amount of sulfur accommodated on the catalyst (S-total). DBT conversion and Stotal increased linearly with Ru content. In a second part, labile sulfur amount was also determined under the catalyst working conditions and different results were obtained. Indeed, when the catalysts were marked with [S-35] and with [S-35]DBT under HDS reaction conditions, the obtained labile sulfur quantities (S-0A) were significantly higher than the ones measured during the presulfidation stage (SO). These results showed that the labile sulfur is not formed on RuCs catalysts until the HDS reaction proceeds, which is quite different from that reported before for Mo, Pt, or Pd systems. (C) 2003 Elsevier Science (USA). All rights reserved.