The sulfidation of Al2O3- and ASA-supported NiW catalysts under conditions relevant to industrial practice was studied for the first time by W-182 Mossbauer spectroscopy. Only limited number of 182W Mossbauer experiments have been performed previously, mainly due to the need of having specialized equipment for such measurements. W-182 MAS can clearly distinguish between WO3- and WS2-type phases encountered in the calcined and sulfided forms of W-based hydrotreating catalysts. NiW/Al2O3 catalysts are more difficult to sulfide than their Mo-based counterparts and, hence, intermediate stages of sulfidation can be studied as separate phases. At low sulfidation rates, an intermediate WS3-type phase is identified at temperatures as high as 673 K. The presence of Ni atoms facilitates sulfidation and in this case the formation of an oxysulfidic intermediate is observed. Sulfidation at 673 K and 0.1 MPa leads to a poorly crystalline WS2 phase, whereas subsequent sulfidation at 4.0 MPa results in the development of better-defined WS2 structures. The merits of W-182 Mossbauer spectroscopy to study W-based hydrotreating catalysts are discussed. (C) 2009 Elsevier B.V. All rights reserved.