The performance of gas oil hydrodesulfurization catalysts was successfully improved through the modification of oxidic precursors with thioglycolic acid. XPS study indicates that addition of this chelating agent affects the sulfidation of the supported metals. The higher catalytic performance was attributed to an optimization of the nature and morphology of the active phase due to the use of this chelating agent which enables a simultaneous sulfidation of both Co and Mo atoms. (C) 2007 Elsevier B.V. All rights reserved.