The influence of the preparation method and sulfidation conditions on the structure and activity of ASA-supported NiW catalysts was investigated by a combination of Co-57 Mossbauer emission spectroscopy (MES), transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), and thiophene hydrodesulfurization (HDS) activity measurements. Ni is sulfided already at low temperatures. This nickel sulfide phase redisperses at higher temperatures over the edges Of WS2 particles to form "Ni-W-S"-type phases. The formation of such highly active phases is facilitated by the partial transformation of intermediate WOxSy phases to WS2 (W L-III edge XAS) at 673 K. In addition to this "Co-Mo-S" analogue nickel sulfide particles are present in dispersed form close to an oxysulfidic tungsten phase. It is found that the sulfidation of NiW/ASA closely resembles that of NiW/Al2O3. Trends in the HDS activity as a function of catalyst pretreatment were evaluated. A higher calcination temperature (from 673 to 823 K) decreases the HDS performance stemming from a lower W sulfidation degree and a more dominant presence of small oxysulfidic tungsten particles. Increasing the sulfidation pressure from atmospheric pressure to 15 bar leads to a strong increase of the HDS activity. Whereas sulfidation at 923 K results in a well-crystallized WS2 phase (XAS), the concomitant loss in dispersion (TEM) is detrimental to its performance. Moreover, indications are found that more complete crystallization of the WS2 phase results in a lower activity. (C) 2004 Published by Elsevier Inc.