A series of Ni-Mo/Al2O3 catalysts for hydrotreating deasphalted vacuum bottoms has been investigated after aging in a commercial reactor during a typical industrial run. This series has been conformed by catalysts with different pore size distribution, active metal loading, and pellet size.The aged catalysts retained metals from the hydrotreated charge in different amounts depending on their physicochemical characteristics. Based on the theory that only a fraction of the active area of the catalysts was accessible for the metallic molecules it was possible to determine that the active area is located in pores of a minimum pore size diameter of approximately 104 Angstrom. The current contribution shows how this type of comparative study can be used to identify the most appropriate catalyst design for a given set of reaction conditions and feedstock quality. (C) 2003 Elsevier B.V. All rights reserved.