The sulfidation of oxidic mixed metal catalysts is an important step in the activation of these catalyst precursors for hydrotreating reactions. The sulfidation of NiW/gamma -Al2O3 catalysts is especially interesting since it is possible to study intermediate stages of sulfidation as separate, stable phases. By carrying out a systematic study, a detailed picture was obtained of the sulfiding reactions and the species formed. The major techniques used were FTIR(NO) and semiquantitative quasi in situ XPS. A reference catalyst was obtained by extraction of Ni from the oxidic catalysts. Four types of Ni are present in the oxidic NiW/gamma -Al2O3 catalysts, viz., Ni in a surface aluminate, Ni in a mixed oxide with W,Ni in a mixed oxide with W and Al, and bulk Ni aluminate. No separate Ni oxide phase is present. It was found that even in dried NiW/gamma -Al2O3 catalysts, a significant part of Ni strongly interacts with either W or the support and is present in subsurface positions. This subsurface Ni species migrates to the surface upon sulfidation below 373 K. Sulfidation below 473 K shows the development of a Ni sulfide species, which is in close interaction with an oxidic or partially sulfided W6+ phase. In addition, part of the W phase can be sulfided at low temperature to form WS3 and it was established that no W4+ species are formed below 600 K. XPS indicated that sulfidation above 600 K results in a distinct change in the chemical environment of Ni ions, which was assigned to the formation of the so-called NiWS phase. Remarkably, FTIR(NO) showed that after sulfidation above 700 K, the gas-phase-exposed W4+ sites become largely inaccessible for NO. This is interpreted as a full decoration of WS2 edges, probably with Ni sulfide, which occurs at a significantly higher temperature than the initial formation of NiWS. (C) 2000 Academic Press.