The aim of this work was to investigate vacuum residues hydroconversion using acid-modified catalysts. To achieve this goal, silicon was incorporated by a grafting procedure. The catalysts with different silicon amounts in the support or hydrogenating actives phases were evaluated performing hydrotreatment of a Safaniya vacuum residue at 370 degrees C. A multi-techniques set of analyses was used, in order to characterize both the sulphided active phase and acidity. It can be concluded from this study that acidity is beneficial for conversion of asphaltenes but the hydrogenation active phase does not seem to be the main way to improve the transformation. The proposed mechanism is the following: with short reaction time, acidity promotes cracking of naphthenic and alkyl units but hydrogenation is not completed. Then, with longer reaction times, asphaltenes reveal a loss of aromatic rings which demonstrates that the hydrogenation pathway is enhanced and a significant loss of molecular weight is observed. This new way of transformation opens the way to a complete change of reactivity over the whole HOT processes. If used as a pretreatment of petroleum residues, this type of catalyst could drastically improve the conversion yield of heavy feeds in order to develop sustainable refining processes. (C) 2014 Elsevier B.V. All rights reserved.