The relative hydrogenation activity in typical hydrotreating conditions of toluene, m-xylene, 1,3,5-trimethylbenzene and 1,2,4,5-tetramethylbenzene was unexpectedly found to decrease over a sulfided NiMo/Al2O3 catalyst and to increase over sulfided Ni/Al2O3 and Mo/Al2O3 catalysts. This change of activity trend is tentatively interpreted by the formation of a mixed NiMoS phase with a different electronic state compared to Ni or Mo sulfide phases. The nature of the aromatic compound influences strongly the magnitude of the promotion effect of Ni on the activity of the Mo in the sulfided NiMo/Al2O3 catalyst.