Sulfided CoMo supported on TiO2, Al2O3, or SiO2 was studied by CO adsorption followed by infrared spectroscopy (IR/CO). The inversion of adsorption isotherms obtained by IR spectroscopy (the 2D-IRIS method) was performed on promoted and nonpromoted catalysts to verify that ν(CO) vibration contributions from 2090 to 2000 cm−1 can be attributed to CO/CoMoS interactions, free of the contribution coming from the CO/nonpromoted S-edges of MoS2 nanoparticles. The second derivative of the spectra evidences the presence of three different promoted sites. Promotion of MoS2 by in situ chemical vapor deposition makes it possible to ascribe the different ν(CO/CoMoS) contributions to partially or totally promoted M-edges (2072–2074 cm−1, 2082–2085 cm−1) and partially promoted S-edges (2054–2057 cm−1). Then the IR/CO method highlights Co promotion on both edges (the S-edge and the M-edge) of MoS2 slabs. The hydrodesulfurization activity of these catalysts shows that the total concentration of promoted sites is an important but not unique factor, since the structures of the CoMoS sites modify their intrinsic activity. A parallel with the different CoMoS site amounts reveals that partially promoted M-edge sites of CoMo-supported catalysts are the most active for hydrodesulfurization. This makes IR/CO a direct way to account for the most active types of promoted sites of CoMo supported catalysts.