The impact of hydrotreatment catalysts acid properties in residues hydrotreatment activities (hydrodemetallization (HDM), hydrodesulfurization (HDS), hydrodeasphaltenization (HDAsC7), hydro-denitrogenation (HDN) and Conradson carbon conversion (HDCCR)) was evaluated. NiMoP based catalysts carried on multimodal/bimodal alumina were used. Within this framework, the acid properties of alumina carriers were modified (in terms of nature, number and strength of acid sites) in a wide range of acidity. This was achieved by doping the carriers using different additives (fluorine, SiO2, zeolite) at different contents introduced by impregnation (for F and SiO2) or co-mixing (for SiO2 and zeolite). Acidity additives effect on sulfide active phase was followed by means of a toluene hydrogenation catalytic test (to characterize the hydrogenating function), transmission electron microscopy and infrared spectroscopy using CO as probe molecule. Sulfided catalysts acidity was quantified through a cyclohexane isomerization catalytic test. The distribution of Bronsted and Lewis acid sites were distinguished by infrared spectroscopy using respectively lutidine and pyridine as probe molecules. It was shown that only fluorine introduced after the active metals impregnation, allowed to develop the most enhanced Bronsted acidity without a major impact in the active phase. A catalytic test was performed in a lab scale batch reactor at hydrotreatment conditions and using a Safaniya vacuum residue feedstock. Hydrotreatment functions were followed showing that among the acidity additives that were used, fluorine enhanced most the asphaltenes conversion. This was attributed to the higher number of strong Bronsted acid sites. (C) 2010 Elsevier B.V. All rights reserved.