The effect of the impregnation method (co-impregnation and sequential) and the incorporation order of the active phases, Ni and Mo, on the structure and catalytic activity of NiMo hydrotreating catalysts supported on an Al-pillared montmorillonite has been investigated. The catalysts were characterised. by X-ray diffraction, N-2 adsorption, thermogravimetric analysis, temperature-programmed reduction, UV-Vis diffuse reflectance, in situ laser Raman, and X-ray photoelectron spectroscopies, and their catalytic behaviour tested in the hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN) of gas oil. It was found that the catalyst prepared by co-impregnation of Ni and Mo was more active than those prepared by sequential impregnation, and of the latter ones, the most active was that in which Mo was impregnated prior to Ni. Characterisation results revealed that by co-impregnation, the dispersion of the Mo phase was significantly increased due to the presence of Ni and, on the other hand, more Ni stay together to Mo on the external surface of the clay, favouring thus the formation of Ni-Mo-O interaction species, precursors of the "Ni-Mo-S" active phase. The relatively low activity of the sequentially impregnated Mo-Ni/Al-PILC catalyst was due to the absence of synergism between Ni and Mo because most of Ni was as Ni2+ ions into the octahedral layers of the clay, separated from the Mo phase. All NiMo bimetallic catalysts exhibited initially a better selectivity towards HDN than towards HDS reaction, probably due to their acidity properties. (C) 2003 Elsevier B.V. All rights reserved.