Alumina supported MoW hydrotreating catalyst was synthesized by using bimetallic H 4 [SiMo 3 W 9 O 40 ] Keggin heteropolyacid (HPA). Catalysts based on monometallic H 4 [SiMo 12 O 40 ] and H 4 [SiW 12 O 40 ] and their mixture were also studied. Genesis of the active phase was studied during atmospheric gas sulfidation by H 2 S/H 2 of the catalysts by in-situ Quick X-ray absorption spectroscopy (XAS) and High-Angle Annular Dark-Field (HAADF) imaging. The combination of different chemometric tools such as Principal Component Analysis (PCA) and Multivariate Curve Resolution with Alternating Least Squares (MCR-ALS) allowed to determine the number of intermediate species, their chemical nature and concentration profiles during sulfidation. It was found that tungsten sulfidation using bimetallic HPA precursor started at lower temperature, compared to W sulfidation in the monometallic and in the mixture of monometallic HPA catalysts. Simultaneous sulfidation of Mo and W atoms in case of the bimetallic molecular precursor can govern the formation of mixed MoWS 2 phase, which formation during activation was evidenced by HAADF.