Regeneration of hydrodesulfurization (HDS) catalysts is an indispensable step for restoring the activity of used industrial Co(Ni)Mo/Al2O3 solids. However, controlling the regeneration step is critical for the last generation of HDS catalysts leading to a non-complete recovery of the activity. Addition of organic additives is a solution to reach the level of activity required for these regenerated catalysts. For this purpose, the use of an additive, maleic acid, has been studied to activate a regenerated CoMo/Al2O3 catalyst. Influence of maleic acid has been determined at different steps of the preparation: at the oxide state, after extraction of the solids or liquids resulting from the interaction of maleic acid with oxide species at the surface of the solid, and finally at the sulfide state. Solids (or liquids) were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), UV-visible and Raman spectroscopies, ex situ and in situ X-ray diffraction, (27)AI and C-13 nuclear magnetic resonance (NMR) spectroscopy, and X-ray photoelectron (XPS) spectroscopy. Finally, catalytic activity evaluation was performed in the HDS of both 4,6-dimethyldibenzothiophene (4,6-DMDBT) and a straight-run gasoil (SRGO). Results emphasized the efficacy of maleic acid to extract Co particularly if coming from cobalt molybdate, CoMoO4 species forming a cobalt maleate complex. After sulfidation, the addition of maleic acid allows a more rapid sulfidation of Mo while Co is not released significantly from the cobalt maleate complex before 300 degrees C. This leads to the formation of a Co-promoted MOS2 phase in a higher proportion enhancing HDS catalytic properties. (C) 2015 Elsevier Inc. All rights reserved.