In situ sulfidation of a commercial alumina-supported CoMo hydrotreating catalyst (3 wt% Co, 12.3 wt% Mo) has been studied by QEXAFS at Co K-edge. Sulfidation was performed by heating progressively the oxide precursor from room temperature (RT) up to 673 K in a H-2/10% H2S pow tramp 4 K/min). XAFS spectra were recorded each 10 K with an acquisition time of only 30 s. The obtained XANES and EXAFS data were compared with those of a Co/Al2O3 (2.36 wt% Co) used as reference sample. It is evidenced that sulfidation of Co atoms in a CoMo/Al2O3 catalyst starts at room temperature while the sulfidation of a Co/Al2O3 catalyst begins at 473 K.