In this study, the influence EDTA on the reducibility and metal-support interactions of SiO2-Al2O3 supported Ni, Mo and NiMo catalysts in the dried (uncalcined) and calcined states were investigated using temperature programmed reduction (TPR) technique. EDTA addition delayed the reduction of Ni and caused a remarkable increase (186 degrees C) in the reduction temperature in the uncalcined state, whereas after calcination both types of Ni/SiO2-Al2O3 catalysts (with and without EDTA) showed similar reduction behavior. For the Mo/SiO2-Al2O3 catalyst, the presence of EDTA reduced strong interaction between the Mo species and the support in both the uncalcined and calcined states and caused easy reduction of all Mo species at a low temperature region (450-550 degrees C) whereas, the catalyst prepared without EDTA contained a significant amount of Mo species strongly bound to the support that required higher temperatures (>800 degrees C) for reduction. In the uncalcined NiMo/SiO2-Al2O3 prepared without EDTA. Ni reduction occurred first at a substantially lower temperature before Mo reduction, while with EDTA, Mo reduction was closely followed by Ni reduction in the temperature region of 400-500 degrees C. Mo and Ni species strongly bound to the support were substantially low and both the promoter (Ni) and Mo were present in close association in the support surface in both the uncalcined and calcined forms of the catalyst prepared with EDTA, compared to that prepared without EDTA. The catalyst prepared with the addition of EDTA to the impregnation solution was remarkably more active than the catalyst prepared without EDTA both in the calcined and uncalcined forms. Weaker interaction between the support and the active phase precursors (oxidic phases of Mo and Ni) played a prominent role in the HDS activity enhancement of catalysts prepared with chelating agents, particularly, when sulfidation was carried out after calcination. (C) 2011 Elsevier B.V. All rights reserved.