The effect of high-pressure sulfidation on (Co)Mo/Al2O3 catalysts was studied by means of IR spectroscopy of adsorbed CO and thiophene hydrodesulfurization (HDS). A new IR cell, called CellEx, was designed in order to characterize catalysts sulfided in situ under H2S/H-2 flow with pressure varying from 0.1 up to 4.0 MPa. On Mo/Al2O3, high sulfidation pressure changes neither the HDS rate nor the MoS2 site concentration. Conversely on CoMo catalyst, sulfidation under high pressure leads to a marked increase of the thiophene HDS activity and of the Co-promoted site concentration as well as to some changes in the local structure of CoMoS sites.