MThe isotopic exchange has been studied between catalyst radiosulfur and H2S, formed in thiophene hydrodesulfurization (HDS) (named S-displace) on alumina supported molybdena, on CoMoOx, PdMoOx, PtMoOx and on silica-alumina supported NiWOx. S-displace was compared with radiosulfur exchange data between catalyst radiosulfur and gas phase H2S (S-exc) determined previously. The extent of S-exc was higher than that of the S-displace for Mo, CoMo in and NiW, whereas the extent of S-displace from PdMoO and PtMoO was significantly higher, than that of S-exc. Thiophene HDS product distribution data are discussed in terms of increased C=C hydrogenation and C-C hydrogenolysis activity, explained by increasing H2S production with longer circulation time of the thiophene/H-2 mixture, The C-1/C-3< 1 ratios among C-4-hydrogenolysis products indicate some coke formation. The decrease of thiophene HDS activity is presumably a consequence of increasing site-blocking with the formation of more H2S and coke with longer duration of thiophene treatment.