Chelating agents enhance the catalytic properties of HDT catalysts. It was evidenced that the presence of this organic compounds delays sulfidation. In the present study, we focused on the interaction of the chelating agent with industrial CoMo on alumina HDT catalysts after impregnation with a diammoniurn salt of ethylenediaminetetraacetate (diA-EDTA), drying or drying and calcination. X-ray diffraction (XRD) and Raman spectroscopy characterization demonstrate the strong interaction of the chelating agent with CoMoO4. After sulfidation, enhanced catalytic activities are obtained for the treated catalysts. Interaction of model CoMoO4 and CoAl2O4 Solids with a solution of diA-EDTA demonstrates that the chelating agent is able to solubilize Co2+ cations from the molybdate, but not from the spinet structure. (C) 2007 Elsevier B.V. All rights reserved.