Silica-supported NiWS catalysts with a high activity for thiophene hydrodesulfurization are obtained when chelating agents such as 1,2-cyclohexanediamine-N,N,N',N'-tetraacetic acid (CyDTA), ethylenediaminetetraacetic acid, or nitrilotriacetic acid are added in the impregnation stage. X-ray photoelectron spectroscopy has been used to follow the state of Ni and W during the temperature programmed sulfidation of Ni-W model catalysts prepared with and without chelating agents on planar SiO2 films on silicon substrates. Fully sulfided catalysts have been tested in thiophene hydrodesulfurization. The activity increases with increasing Ni content and reaches a plateau at a Ni:W atomic ratio of 0.66. In NiW catalysts prepared without additives the sulfidation of Ni precedes that of W. However, Ni sulfide formed at low temperatures changes its structure at high temperatures where WS2 is present, as indicated by the Ni XPS binding energy, which we tentatively attribute to redispersion of sulfidic Ni over WS2. Chelating agents stabilize Ni against sulfide formation at low temperature, the effect being strongest when CyDTA. is applied. CyDTA retards the sulfidation of Ni to temperatures where all W has already been sulfided. This complete reversal in the order in which the two elements convert to sulfides is seen as the key step in preparing highly active NiWS HDS catalysts.