By using radioisotopic testing technique a poisoning effect of N-containing compounds on sulfided-(S-35) (Co)Mo catalysts in the reaction of thiophene hydrodesulfurization (HDS) has been studied. In the course of the experiments the number, distribution and reactivity of surface SH groups of various types and related functioning vacancies as well as catalytic activity of poisoned and non-poisoned Mo and CoMo catalysts have been measured. It has been shown that desulfurization (DS) and hydrogenation (HYD) reactions mostly proceed on the same sites. These sites are attributed as "rapid" and belong to Mo. The results have been compared with the literature data of the HDS of dibezothiophene (DBT) and its methyl derivatives. The comparison has shown that in the course of the HDS of thiophene and 4-methyl- and 4,6-dimethyldibenzothiophenes CoMoS catalysts demonstrate stronger resistance to poisoning in HYD reactions than DS ones. In the course of dibezothiophene HDS, catalysts demonstrate the opposite trend. It has been suggested that the DS of thiophene and alkyl derivatives of DBT and all the HYD reactions should proceed on Mo related sites while the DS of DBT-on Co related sites. The HYD reactions of the DS products of DBT should take place on Mo related sites. A method to evaluate the efficiency of (Co)MoS catalysts for HDS of various types of crudes has been developed. (C) 2003 Elsevier Science B.V. All rights reserved.