Infrared and volumetric studies of NO adsorption have been used to elucidate the type of surface structures present in sulfided Co Al 2 O 3 , Ni Al 2 O 3 , Mo Al 2 O 3 , CoMo Al 2 O 3 , and NiMo Al 2 O 3 catalysts. Catalytic implications were obtained from measurements of thiophene hydrodesulfurization (HDS) activities. The content of active material in the catalysts and calcination temperature were varied in these studies. The ir bands were observed to be very different for NO adsorbed on Co, Ni, and Mo atoms and are sensitive to the surface concentration of the element, the nature of the surface phase and the extent of sulfiding or reduction. For Mo Al 2 O 3 catalysts, the NO most probably adsorbs on edge or corner sites of MoS 2-like structures and the adsorption therefore reflects the edge dispersion of these structures. In the case of the promoted CoMo Al 2 O 3 and NiMo Al 2 O 3 catalysts, the ir studies give simultaneous information on the NO adsorption occurring on the Co or Ni promoter atoms and that occurring on the Mo atoms. It is seen that the addition of promoter atoms results in a decreased adsorption on the Mo atoms. This indicates that the promoter atoms occupy edge positions of the MoS 2 “support”. The Co atoms located in such positions are found to be related to those present in the so-called CoMoS structure identified previously by Mössbauer emission spectroscopy. Evidence for similar Ni-Mo-S type structures is found. The HDS activity correlated neither with the total amount of chemisorbed NO nor with the amount of NO adsorbed on the Mo atoms. However, for all the catalysts a good correlation was observed between the HDS activity and the amount of NO adsorbed on the Co or Ni promoter atoms. This further supports the “CoMoS model” in which the primary role of the promoter atoms is to create new sites (associated with the promoter atoms) with higher intrinsic HDS activity than that of the unpromoted sites. Pyridine, which is known to be a partial HDS poison, was observed to block a large fraction of the NO adsorption sites.