The sulfidation mechanism of supported NiW hydrotreating catalysts is described in detail based on literature data and experiments for carbon and alumina-supported NiW catalysts. It is more difficult to convert tungsten oxides to WS2 than molybdenum oxides to MoS2 and therefore typical sulfidation procedures tend to result in oxysulfidic tungsten phases (WOxSy) next to the layered WS2 phase. The former phase appears to stabilize rather dispersed Ni sulfide particles (NiS-WOxSy) which is found to be a precursor to the 'Ni-W-S' phase: Mossbauer spectroscopy clearly shows that the NiS particles redisperse over the WS2 edges when partly oxidic tungsten phases transform to WS2 slabs. As a consequence of very slow W sulfidation, the Ni promoter ion can end up in two phases, i.e. 'Ni-W-S'-type and in NiS-WOxSy, phases, this in contrast to Co(Ni)Mo catalysts that mainly contain 'Co(Ni)-Mo-S' phases after typical sulfidation procedures. The NiS-WOxSy phase is known to perform well in liquid-phase hydrodesulfurization (HDS) reactions whereas the 'Ni-W-S' phase is most active for gas-phase HDS. The relative speciation of these two phases for alumina-supported NiW can be controlled by parameters as the calcination temperature and the sulfidation temperature and pressure. Notably, elevated sulfidation pressures are preferred over elevated sulfidation temperatures because in the latter case the WS2 slabs tend to sinter and crystallize which negatively affects the performance. This work shows the existence of a strong support effect for carbon- and alumina-supported NiW. Similar trends were found for amorphous-silica-alumina-supported NiW which is consistent with the finding that the sulfidation mechanism is similar to that of NiW/Al2O3. Even when WS2 sulfidation is completed, as for instance found for a carbon-supported NiW catalyst at 673 K, a significant fraction of Ni is not involved in the 'Ni-W-S' phase. Chelating agents improve the edge occupation in NiW catalysts significantly by retarding Ni sulfidation. The result is a more efficient formation of the 'Ni-W-S' phase and optimized catalysts have a significantly higher activity than conventional ones. (c) 2007 Elsevier B.V. All rights reserved.