NiW/Al2O3 catalysts with constant tungsten loading and variable amounts of nickel were tested in the hydrodesulfurization (HDS) of thiophene and 4,6-dimethyldibenzothiophene (4,6-DMDBT), and in the isomerization of o-xylene. Introduction of Ni promotes the overall activity for both thiophene HDS and 4,6-DMDBT HDS, but the synergy is more pronounced for the former. Such effect has been analyzed on the basis of the two reaction routes generally involved in the HDS mechanism: direct desulfurization (DDS) and hydrogenation (HYD). Moreover there is a parallelism between the increase of the DDS route and the isomerization of o-xylene, proof of a correlation between the promoter content and some acidic properties of the sulfides. HREM examinations indicated few effect of the promoter over the morphology of the sulfide phase and XPS studies show that Ni is mainly present in the form of "NiWS" phase at the low Ni/[Ni + W] ratio, while separate Ni sulfide is formed at the high one. Such an evolution of Ni species varying with Ni content is responsible to the changes in catalytic activity and selectivity. (C) 2003 Elsevier B.V. All rights reserved.