The objective of this paper is to establish relations between the presence of particular surface species in sulfided NiW/gamma -Al2O3 catalysts and their catalytic activity in hydrosulferization (HDS) reactions. It was found that calcination and sulfidation temperatures influence selectivity and activity. The relationships found are quite different for the two reaction systems studied, i.e., gasphase thiophene HDS and liquid-phase dibenzothiophene HDS. In the latter reaction, a high-temperature calcination at 823 K followed by a sulfidation up to 673 K results in a higher selectivity for hydrogenated products. Catalysts sulfided up to 673 K have a higher activity in liquid-phase dibenzothiophene HDS than those sulfided above 673 K. In contrast, catalysts sulfided up to 673 K have a relatively low activity for gas-phase thiophene HDS compared to those sulfided at 823 K. After high-temperature sulfidation the differences in the activity due to the preceding calcination step disappear. It is proposed that in low-temperature sulfided NiW/gamma -Al2O3 catalysts the active phase consists of a highly disperse Ni sulfide species, in close interaction with an oxysulfidic W6+ phase. In NiW/gamma -Al2O3 catalysts sulfided above 750 K, the active phase consists Of WS2 slabs decorated with Ni sulfide and is characterized as Type I NiWS.