The effect of nickel concentration and sulfiding temperature on the type and distribution of the surface species and on the dispersion in a series of alumina-supported Ni-W hydrodesulfurization (HDS) catalysts were investigated by temperature programmed reduction (TPR), diffuse reflectance spectroscopy (DRS), dynamic O-2 and NO chemisorption (DOG, DNOC), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), sulfur content analysis and thiophene HDS reaction. In the thiophene HDS reaction, the Ni-W/gamma-Al2O3 system showed a strong synergistic behavior. The maximum activity was achieved at about Ni/(Ni + W) = 0.6. When sulfiding temperature increased from 673 K to 1073 K, the activity monotonously decreased with the S-shaped curve. TPR results revealed three different nickel species, a highly dispersed NiO surface species, a mixed Ni-W oxide phase, and a subsurface NiAl2O4-like phase. The nature and the amount of these species depended on nickel loading. DRS results provided complementary support for the presence of these species. The results of XRD, sulfur content analysis, DOG, DNOC, and XPS indicated that the sulfidability and the crystallinity of WS2 increased with the increase in sulfidation temperature. The surface nickel species diffused into the bulk at high sulfidation temperature.