The changes occurring in the alumina phase during hydrothermal treatment of gamma-alumina in the presence and absence of additives such as P, F, phenol and acetic acid, were investigated with a view to understand the mechanism of pore widening in gamma-alumina under hydrothermal conditions. The results showed that the formation and growth of boehmite into large crystallites by rehydration of gamma-alumina was responsible for pore enlargement. As a part of the study, a wide pore Ni-Mo/gamma-alumina was prepared by hydrothermal treatment procedure and its performance in vacuum residue hydroprocessing was evaluated. The catalyst showed a remarkably high activity for hydrodemetallization and asphaltenes conversion reactions in vacuum residue hydroprocessing. Distribution of the deposited metals within the catalyst pellet was more uniform for this catalyst compared to that of a conventional HDM catalyst. (C) 2002 Elsevier Science B.V. All rights reserved.