Surface science models of silica-supported NiMo catalysts have been prepared to study the formation of the active phase ("NiMoS") in hydrotreating catalysts. Combination of XPS and thiophene hydrodesulfurization (HDS) measurements shows that the key step in the formation of the NiMoS phase is the order in which Ni and Mo precursors transfer to the sulfidic state. In NiMo systems prepared by conventional methods the sulfidation of Ni precedes that of Mo. However, complexing Ni to chelating agents like nitrilotriacetic acid, ethylenediamine, ethylenediaminetetraacetic acid (EDTA), and 1,2-cyclohexanediaminetetraacetic acid (CyDTA) retard the sulfidation of Ni. For EDTA and CyDTA the Ni sulfidation is delayed to temperatures where MoS2 is already formed. These catalysts show the highest activity in thiophene HDS, indicating that complete sulfidation of Mo preceding that of Ni provides the ideal circumstances for NiMoS formation. (C) 2001 Academic Press