Activation of hydrodesulfurization (HDS) catalysts through sulfidation of oxide precursors is a critical procedure required to achieve preparation of the Co(Ni)-promoted Mo(W)S-2 sulfides commonly used for performing sulfur removal from transportation fuels. Better optimized preparation of HDS catalysts can be reached if organic additives, glycol-type compounds or chelating agents, are used during the activation process. Moreover, comprehension of the role of the organic additives during activation has become even more essential with the increasing necessity to regenerate satisfactorily the last generation of HDS catalysts, more sensitive to the harsh conditions used during the oxidative regeneration treatment. In this respect, maleic acid (MA) was recently found to be an efficient additive for the activation of regenerated HDS catalysts. In the present study, deeper understanding of the role played by maleic acid for restoring the HDS activity was determined at different steps of the preparation of a regenerated CoMo-Al2O3 (here called CoMo-R) catalyst. Comparison was first performed to two other additives, a chelating agent, ethylenediaminetetraacetic acid (EDTA) and a glycol-type compound, triethyleneglycol (TEG), after impregnation and drying steps, using UV-vis diffuse reflectance spectroscopy, ex situ and in situ X-ray diffraction and thermal analysis. Results emphasized that maleic acid is the most efficient additive for extracting cobalt from cobalt molybdate species leading after sulfidation to much better HDS catalytic properties in the HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) and of a straight run gasoil. The sulfidation step was also carefully analyzed during the activation of CoMo-R in the presence of MA using inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray photoelectron, Raman and electron paramagnetic resonance spectroscopies, mass spectrometry analysis and transmission electron microscopy. This detailed analysis allows proposing a complete description of the sulfidation mechanism in the presence of MA leading to a better understanding of the key aspects to be reached during the activation process.