The support effect was analyzed for hydrodesulfurization CoMo catalysts prepared with EDTA as the chelating agent. Supports with different isoelectric points, gamma-Al2O3, Al2O3-MgO, and SiO2, were used. Different sequences of EDTA, Co, and Mo impregnation were used to analyze whether the sequence of addition affected catalytic activity. Changing the order of addition of Co, Mo, and EDTA produced only a small effect on the catalyst HDS activity. The simultaneous impregnation of Co, Mo, and EDTA led to higher HDS activity than sequential impregnations. In contrast, changing the support nature produced important variations in catalytic activity, following the increasing order: Al2O3 < SiO2 < Al2O3-MgO. It was found that the changes in HDS activity with the type of support are related to different levels of promotion of Mo by Co, and to changes in the sultidation patterns induced by different interactions of the active phase precursors with each support. (c) 2006 Elsevier Inc. All rights reserved.