A series of Ni-Mo/Al2O3 catalysts for the hydrotreating of deasphalted vacuum bottoms has been investigated as fresh catalysts and after aging in an industrial reactor during a typical commercial run. Several structural parameters have been varied in the catalyst series. In particular, pore size, surface area, active metal loading and pellet size were changed to investigate the role of kinetic and diffusional phenomena present in this process. The samples were placed in different locations along the industrial reactor allowing for a comparative analysis of the different samples under varying environments, since along the reactor, quality, temperature, and H-2 partial pressure continuously change. Fresh and aged samples were analyzed by a variety of techniques, which include atomic adsorption, scanning electron microscopy, X-ray microanalysis, BET, thermogravimetry and temperature-programmed oxidation. In addition, a laboratory trickle-bed reactor was used to compare the catalytic activity of fresh samples with the residual activity retained by the aged samples after the commercial run. These catalysts deactivate by accumulation of metals and, to a lesser extent, coke deposition. These effects are governed by a competition of kinetic and diffusional phenomena. The kinetic phenomena, which involve adsorption and decomposition of metal-bearing compounds are mainly related to the active metal content and overall surface area of the catalyst, while the diffusion of the molecules inside the pellet are controlled by pore size distribution and pellet size. The current contribution shows how this type of comparative study can be used to identify the most appropriate catalyst configuration for a given set of reaction conditions and feedstock quality. (C) 2000 Elsevier Science B.V. All rights reserved.