Sulfurization remarkably affects the physical and chemical properties of dispersed catalysts. In this study, molybdate is taken as catalyst precursor for slurry-bed hydroprocessing of residuum to study the mechanics of sulfurization. Several conditions are used to evaluate the activities of the sulfurized catalysts, and XRD, TEM and BET are employed to characterize the sulfurized catalysts. The results show that molybdate is firstly transferred into some kind of crystallized oxothiomolybdate and then mainly into micro-crystallized MoS2 in the end. The dispersivity of the MoS2 granules is greatly enhanced compared to the oxothiomolybdate. XPS analysis shows the portion of Mo-1V in molybdenum species on the surface of the catalysts is enhanced by the addition of NH4Cl during sulfurization. Therefore, the catalytic activity, measured using anthracene as chemical probe in the practical reaction system of hydroprocessing for residue, is also enhanced. (C) 2004 Elsevier B.V. All rights reserved.