Currently the production of low sulfur diesel fuel (< 0.05 wt.% S) is an important topic in oil refineries due to new environmental legislation. Consequently, alternative catalysts for deep HDS processes are required and NiW/gamma-Al2O3 catalysts are a promising option. An important issue in the application of NiW/gamma-Al2O3 catalysts is their activation. In this work a quantitative description of the sulfidation mechanism of W in NiW/gamma-Al2O3 catalysts, as obtained from temperature programmed sulfidation measurements, is presented. It is shown that the calcination temperature has a great influence on the relative importance of the different sulfidation reactions. Especially the WS2 formation at sulfidation temperatures below 600 K is strongly decreased after calcination above 673 K. Fe-57-MAS shows that re-dispersion of Ni-sulfide species occurs after sulfidation at 573 K for a dried catalyst to form the so-called NiWS phase. The NiWS phase is also formed on calcined catalysts but only at sulfidation temperatures of 673 K. It is demonstrated that the formation of WS2 slabs is a prerequisite for the re-dispersion of Ni-sulfide species.