Flat model systems of supported oxidic COMO hydrodesulfurization (HDS) catalysts, consisting of a conducting substrate with a thin SiO2 or Al2O3 layer on top of which the active catalytic phase is deposited by spin coating, were used to study the influence of the support and the use of chelating agents on the formation of the active CoMoS phase. On Co/Al2O3 catalysts Co can only be partially sulfided due to the strong interaction with Al2O3. In the mixed phase CoMo/Al2O3 system, the presence of Mo facilitates the sulfidation of Co. Due to the interaction of Mo with the support the Co-Al2O3 interaction is blocked. In these catalysts the sulfidation of Co and Mo proceed at the same rate. In the case Of CoMo/SiO2 catalysts the weak interaction with the SiO2 support results in the complete sulfidation of Co at low temperatures, while Mo sulfides at a slower rate. Thiophene HDS activity measurements showed that due to the stronger interaction, Al2O3 is a better support for CoMoS formation. By complexing COMO on Al2O3 to chelating agents, like ethylenediaminetetraacetic acid (EDTA), the sulfidation of Co is retarded while that of Mo is facilitated. Although Mo also forms complexes with EDTA, these complexes are less stable than those of Co. Hence, the sulfidation of Mo precedes that of Co. Due to the absence of interaction with the support of the same sulfidation behavior was observed for both supports. The activity tests showed that these catalysts have the highest activity, irrespective of support.