The effect of the degree of W sulfidation on dispersion, morphology, thiophene hydrodesulfurization (HDS) activity, and promotional behavior for supported Ni(Co)W sulfide catalysts was studied by varying the calcination temperature, sulfidation temperature, sulfidatian pressure, and support. The catalysts were characterized by thiophene HDS, X-ray photoelectron spectroscopy; transmission electron microscopy, extended X-ray absorption fine-structure, and Mossbauer emission measurements. Assuming that sulfidation to a WS2 phase proceeds via a WOxSy phase it can be concluded that the ratio between these two phases depends on the sulfidation conditions and W-support interaction. For the highest thiophene HDS activity, sulfidation conditions that result in nearly maximum sulfidation of the WOxSy phase are necessary. However, one should be aware that complete sulfidation results in the formation of WS2, which is more susceptible to sintering. For Al2O3-supported NiW catalysts preferable sulfidation conditions are a low temperature and a high H2S/H-2 sulfidation pressure. In contrast, for TiO2-supported NiW the calcination and sulfidation temperatures have to be low and sulfidation at atmospheric H2S/H-2 pressure is already sufficient, because W is more easily sulfided due to its weak interaction with TiO2. The degree of sulfidation also affects the promotional behavior. The optimal Ni/W ratio is higher than the optimal Ni/Mo ratio, while in contrast to CoMo catalysts, for CoW no optimal ratio could be found. In addition, the activity of CoW appeared about the same as the sum of the W and Co activities, regardless of the Co content. In the case of NiW and CoW, Ni and Co are sulfided first. As soon as WS2 appears, the NiS particles already formed partially redisperse to form NiWS, while the CoS particles tend to form a separate Co9S8 phase instead of CoWS.