A very rapid deactivation occurs during hydroprocessing of heavy crude oils and residue. In this work, we have studied the causes of deactivation in detail. For this purpose, hydrodesulfurization of thiophene and straight-run gas oil has been studied on various used catalysts obtained after 280 h of pilot plant operation by using heavy crude oils of 21 and 13 degrees API gravity and a residue of 7 degrees API gravity. The results show high degree of catalyst deactivation with a feedstock having high asphaltenes and metal contents. Used catalysts were regenerated by burning of coke, and then tested with HDS of thiophene and straight-run gas oil. The activity of regenerated catalysts regains substantially, indicating that the catalyst is mainly deactivated by coke deposition. One used catalyst was unloaded from three different zones of the pilot reactor. Analyses conducted on these samples indicate high deposition of metals at the top of the reactor and high deposition of coke at the bottom. Radial distribution of metal deposition showed that vanadium is uniformly distributed throughout the catalyst particle. For the fresh catalyst used in this study (15 nm of average pore diameter), it was found that the primary cause of the catalyst deactivation is coke deposition and covering of active sites by vanadium rather than pore mouth blockage. (c) 2012 Elsevier B.V. All rights reserved.