Abstract: The IR spectra of pyridine adsorbed on oxidic and sulfided form of Ni-Mo and Co-Mo catalyst systems were measured. The results showed thatthe OH group played an important role in the dispersion of MoO₃ on thesurface of support and that Ni and Co promotors apparently increased thedispersion of MoO₃ on the support. Kinetic results and the TGA and IRspectra indicated that a certain site of Lewis type,not of Brosted,may playan important role in the HDN reaction of quinoline.