The main objective of High-Pressure Sulfidation of Hydrotreating Catalysts is to understand the effect of the sulfidation pressure on the active phase structure in CoMo and NiW catalysts. The influence of the sulfidation pressure on the structure and activity of calcined CoMo/Al2O3 catalysts were examined through multiple experiments: to better understand the influence of calcinations conditions on the performance of CoMo/Al2O3 catalysts, a set of uncalcined catalysts with different Co/Mo ratios is investigated. Also the applicability of the Co-Mo-S structural model and the stability of the Co-containing phases for carbon-supported catalysts activated under high-pressure conditions were evaluated. Nitrilotriacetic acid (NTA) and phosphorus containing CoMo catalysts are examined to better comprehend the influence of high-pressure sulfidation of CoMo/Al2O3 catalysts prepared via non-conventional routes. Furthermore, an attempt is made to understand the strongly varying DBT HDS activities among the various catalysts presented in earlier chapters. Also, it is described how the sulfidation of Al2O3- and amorphous-silica-alumina-supported NiW catalysts under conditions relevant to industrial practice using ¹W Mössbauer spectroscopy for the first time. Overall, two major factors influencing the HDS activities of CoMo hydrotreating catalysts are identified: the significant formation of CoSx structures in catalysts with weak metal-support interaction and the existence of a continuum of Type I/II Co-Mo-S structures, where the fully Type II end member is more difficult to reach than previously suspected.