This work analyzes the changes in hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodeasphaltenization (HDAs) and hydrodemetallization (HDM) activities, when a NiMo/TiO2/Al2O3-catalyst is evaluated under three different feedstocks containing heavy Maya petroleum, i.e., Maya-diesel (M-D), hydrodemetallized Maya-diesel (HM-D) and hydrodemetallized Maya-naphtha (HM-N). A NiMo/Al2O3 catalyst was also tested using HM-N as feed, in order to enquire on the effect of changing the nature of the support from Al2O3 to TiO2/Al2O3. The results show that the choice of diluent when performing catalytic activity tests for the ranking of catalyst formulations for hydrotreatment of heavy petroleum like Maya is critical. The incorporation of Ti to the support of the conventional NiMo/Al2O3 catalyst improves the functionalities of the catalyst (HDS, HDN, HDAs), possibly due to Ti-enhanced hydrogenating and hydrodesulfurization functions. The number of coordinatively unsaturated sites (CUS) on the surface of the sulfided phase was higher in the Ti-containing catalyst. (C) 2004 Elsevier B.V. All rights reserved.