The interaction of triethylene glycol (TEG) with alumina and its role in preparing improved NiMo/Al2O3 hydrodesulfurization catalysts was investigated by spectroscopic methods. The FT-IR study shows that TEG is mainly adsorbed on the corners and edges of the alumina microcrystals where the strongest Lewis sites and the higher vOH frequency hydroxyl groups are mainly located. It is also observed that the Mo=O stretching vibration of surface molybdenyl groups in the oxide catalyst precursor is shifted down in the presence of TEG, indicating a lower interaction with the alumina surface. The IR spectra of CO adsorbed on the reduced/sulfided NiMo/Al2O3 catalysts confirm that the amount of promoted phase (NiMoS sites) increases in the samples prepared with TEG. Accordingly, the activity measurements in the HDS of 4,6-dimethyldibenzothiophene show that the catalyst prepared with TEG is more active than the one prepared without it. It is proposed that TEG and its decomposition products, formed upon heating (mainly acetates) occupy preferably the strongly interacting edge and corner sites of the alumina crystals, forcing the Mo and Ni species to migrate mainly to the less reactive plane faces. This weakens the metal-support interaction allowing a better sulfidation and, at the same time, favoring the Ni-Mo interaction and the formation of the promoted NiMoS phase. (C) 2014 Elsevier B.V. All rights reserved.