Co K-edge XANES results confirmed the previous findings that a selective formation of Co atoms in a CoMoS phase is achieved by means of a CVD technique using CO(CO)(3)NO. If MoS2/Al2O3 is exposed to CO(CO)(3)NO vapor and subsequently sulfided, the resultant catalyst exhibits the maximum potential activity of Co-Mo/Al2O3 catalysts based on the MoS2/Al2O3. This activity is obtained when the edges of the MoS2 particles are fully occupied by Co atoms without any blocking by Co sulfide clusters. With Co-MoS2/Al2O3 catalysts prepared by classic impregnation, the edge sites of MoS2 Particles remain unoccupied by Co atoms in a wide range of Co content. In contrast, the edges of MoS2 particles are completely occupied by Ni atoms above 2 wt% Ni. Blocking of the active sites by overlayers of Co(Ni) sulfide clusters decreases the HDS activity to a considerable extent. The CVD technique with CO(CO)(3)NO as the probe molecule provides a novel characterization method for Co(Ni)-Mo sulfide catalysts, giving information about (i) the maximum potential HDS activity, (ii) the extent of blocking of the active phase, (iii) the coverage of Co(Ni) atoms on the edges of the MoS2 particles, and (iv) the dispersion of MoS2 particles in Co(Ni)-MoS2 catalysts. (C) 2003 Elsevier Inc. All rights reserved.