A new analytical cell to perform liquid sulfidation of HDS catalysts in industrial conditions was developed. It enables the alternate recording of time-resolved Mo and Co K edges X-ray Absorption spectra of bimetallic catalysts under high pressure (30 bar) and temperature (350°C) during the same reaction. Thanks to this cell, a comparative study of the species formed upon gas (1 bar, 15% H 2 S/H 2) and liquid sulfidation (30 bar, H 2 /gas oil/4% DMDS) of a CoMoP/Al 2 O 3 HDS catalyst was carried out, together with a discussion on the evolution of the concentration profiles upon activation. Different Mo and Co-based chemical species are involved during gas and liquid sulfidation. On one hand, in industrial liquid sulfidation, polymolybdate species are transformed into depolymerized oxides, then into an oxysulfide, a MoS x species and finally into MoS 2. On the other hand, gas sulfidation skips the depolymerization process because of the immediate supply of H 2 S at the beginning of the process and proceeds under a stepwise transformation of oxide into oxysulfide, itself into MoS 3 species and finally into MoS 2. Cobalt species under gas sulfidation goes through a CoS 2 intermediate whereas in liquid sulfidation, it is a 4-fold coordinated Co oxide species. Irrespective to the activation route, mixture of Co 9 S 8 and CoMoS is obtained at advanced sulfidation stage (T> 200°C).

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