Realistic surface science models of supported molybdenum- and tungsten-based hydro-treating catalysts have been made by impregnating planar thin films of silica and alumina with aqueous solutions of the normally utilized precursor salts of active elements. These models have enabled detailed studies of catalyst preparation and hydrodesulfurization kinetics. Optimization of the catalysts is possible by applying appropriate chelating agents for the promoting elements cobalt and nickel. These complexes work by stabilizing the promoting element such that during sulfidation MoS2 and WS2 structures form before the promoters are released so that these have a higher chance to end up at the edges of MoS2 and WS2 instead of forming the less active Co9S8 and Ni2S3 phases. The extent to which sulfidation of the promoter cobalt is delayed correlates with the activity of the catalyst for thiophene hydrodesulfurization.