In residual oil upgrading by hydroprocessing, two types of reactor technologies namely, fixed-bed and ebullated-bed are commonly used. In this paper the deactivation behavior of catalysts in both types of units is compared based on the information obtained from the analysis of spent catalysts. In the fixed-bed unit, the concentration of vanadium deposits decreased from the entrance (first reactor) to the exit (last reactor), while, carbon content of the catalyst increased from the entrance to the exit. The loss of surface area for the catalysts in various reactors of the fixed-bed atmospheric residue desulfurization (ARDS) unit increased from the entrance to the exit. Distribution of the deposited vanadium with the catalyst pellet was also different in different reactors. Catalysts in the front-end reactors were deactivated mainly by metal deposition, while the back-end catalysts were primarily deactivated by coke deposition. The nature of the coke deposits on different catalysts was found to be significantly different by C-13 NMR. temperature programmed oxidation (TPO) and differential thermal analysis (TGA). Spent catalysts from the ebullated-bed reactor contained a mixture of highly fouled and partly fouled catalysts. The coke on the highly fouled spent catalyst particles was more aromatic and more condensed compared with that on the lightly fouled catalyst particles. Considerable changes in the physical dimensions of the catalyst pellets were noticed for the spent catalysts from the ebullating-bed reactor. In the ebullated-bed reactor, catalyst deactivation was caused not only by deposition of metals and carbon, but also by changes in the physical and mechanical properties of the catalyst under the dynamic conditions of the process. (c) 2006 Elsevier B.V. All rights reserved.