Co-Mo model sulfide catalysts, in which COWS phases are selectively formed, were prepared by means of a CVD technique using CO(CO)(3)NO as a precursor of Co. It is shown by means of XPS, FTIR and NO adsorption that COWS phases form selectively when the Mo content exceeds monolayer loading. A single exposure of MoS2/Al2O3 to a vapor of Co(CO)(3)NO at room temperature fills the edge sites of the MoS2 particles. It is suggested that the maximum potential HDS activity of MoS2/Al2O3 and Co-Mo/Al2O3 catalysts can be predicted by means of Co(CO)(3)NO as a "probe" molecule. An attempt was made to determine the fate of Co(CO)(3)NO adsorbed on MoS2/Al2O3. The effects of the support on Co-Mo sulfide catalysts in HDS and HYD were investigated by use of CVD-Co/MoS2/support catalysts. XPS and NO adsorption showed that model catalysts can also be prepared for SiO2-, TiO2- and ZrO2-supported catalysts by means of the CVD technique. The thiophene HDS activity of CVD-Co/MoS2/Al2O3, CVD-Co/MoS2/TiO2 and CVD-Co/MoS2/Al2O3 is proportional to the amount of Co species interacting with the edge sites of MoS2 particles or COWS phases. It is concluded that the support does not influence the HDS reactivity of CoMoS phases supported on TiO2, ZrO2 and Al2O3. In contrast, CoMoS phases on SiO2 show catalytic features characteristic of CoMoS Type II. With the hydrogenation of butadiene, on the other hand, the Co species on MoS2/TiO2, ZrO2 and SiO2 have the same activity, while the Co species on MoS2/Al2O3 have a higher activity. (C) 2003 Elsevier B.V. All rights reserved.