Ethylenediamine tetraacetic acid (EDTA) and citric acid (CA) were used as organic chelating additives during preparation of NiMo catalysts with wide-pore ZrO2-TiO2 mixed oxides as carrier. A clear beneficial effect of using either chelator was evident when sulfided catalysts were tested in dibenzothiophene hydrodesulfurization (DBT HDS at 320 degrees C and 5.72 MPa, batch reactor) although the optimum concentration of EDTA and CA to obtain materials of maximized activity was different to each other (Ni/EDTA = 1 and Ni/CA = 1/2). These molar ratios seemed to correspond to total impregnated nickel chelation. Augmented chelator concentration resulted in catalysts of decreased hydrodesulfurizating properties probably due to delayed sulfidation of complexated molybdenum that could be formed by reaction of Mo6+ containing species with organic additive excess. in a pseudo-first-order kinetic constant basis, the best catalyst prepared with EDTA had similar HDS activity to that of the optimized formulation with CA (k(HDS) values of 7.8 x 10(-5) and 7.3 x 10(-5) m(3)/kg(cat) s, respectively), both formulations being clearly superior to a NiMo/ZrO2-TiO2 catalyst prepared with no organic additive (k(HDS) similar to 4.7 x 10(-5) m(3)/kg(cat) s). Characterization by thermal analyses (TGA, DTA) and infrared spectroscopy pointed out to delayed Ni2+ sulfidation as the main cause of HDS activity improvement in catalysts prepared in presence of either organic chelator. (c) 2008 Elsevier B.V. All rights reserved.