SBA-15 supported NiMo hydrotreating catalysts with different EDTA/Ni molar ratio were prepared by incipient wetness impregnation method. Hydrotreating activities of these catalysts were studied with Athabasca bitumen derived light gas oil and comparison was done with NiMo/SBA-15 and NiMo/gamma-Al2O3 catalysts. A beneficial effect of chelating ligand was seen in hydrotreating activity and 28% increase in HDS of LGO was found as compared to NiMo/gamma-Al2O3 in the case of EDTA/Ni molar ratio 4. Detailed mechanistic aspect of interaction between support - EDTA (ethylene diamine tetraacetic acid), EDTA-metallic species, support - metal, metallic species - metallic species at different reaction conditions is studied at different process parameters by using different characterization techniques such as X-ray absorption near-edge structure (XANES), Fourier transform infrared spectroscopy (FTIR), HRTEM, XRD, Raman, temperature programmed reduction (TPR), ICP-MS, CO chemisorption and N-2 adsorption. A detailed scheme has been developed to explain the behavior of chelating agent in mesoporous SBA-15 supported catalysts. HRTEM results explain the high dispersion of Ni and MoS2 active phases in sulfided state even though big crystallite of MoO3 was seen in oxide state characterized by XRD and Raman spectra which explains role of EDTA in redistribution of active phases during sulfidation. XANES technique is used as predominant method for atomic level study of various structural changes in sulfided and oxide catalysts to find out the appropriate mechanism of the reaction. Characterization by XANES reveals that the presence of chelating agent delayed Ni2+ sulfidation which is main cause of improvement in HDS and HDN activity of the catalysts prepared in presence of organic chelates. Nickel sulfidation starts only when the EDTA-Ni complex decomposes and released nickel atoms move to the reactive edges of the MoS2 to form a finely dispersed sulfide Ni-Mo-S type II active phase. (C) 2012 Elsevier B.V. All rights reserved.