A series of sulfided NiW/Al2O3 catalysts with a constant W loading and various Ni contents have been prepared by incipient wetness co-impregnation method and characterized by XPS, HREM, TPR and FT-IR-CO chemisorption in order to get a better understanding the nature of the active phases. Characterization results have shown that the Ni promoter preferentially decorates the edges Of WS2 slabs to form the highly active NiWS mixed phases. The significant synergy effect of about 30 times promoted by Ni in the HDS reaction of thiophene is directly correlated to the concentration of NiWS phase. In addition, the addition of Ni induces a slight increase of the WS2 lamellas stacking and a diminution of their lengths, as well as an increasing sulfidability of WOx phase by about 20%. These modifications make rather small contributions to the total HDS activity which cannot explain the drastic promoting effect. The NiWS phase present in sulfided NiW/Al2O3 catalysts was decomposed progressively during TPR process. Reduction pretreatment of one sulfided NiW/Al2O3 catalyst with a Ni/(Ni + W) ratio equal to 0.41 at elevated temperatures leads to a drastic decrease of the thiophene HDS activity, which also clearly demonstrates the progressive destruction of the active NiWS phase. (C) 2004 Elsevier B.V. All rights reserved.