Thiophene hydrodesulfurization and butene hydrogenation have been studied for presulfided MoO3/SiO2 and WO3/SiO2 catalysts using a micro-flow reactor operating at atmospheric pressure. The catalysts have been prepared by dry as well as wet impregnation. The oxidic precursor catalysts were characterized by X-ray diffraction, surface area and pore volume measurements, and temperature-programmed reduction. Catalysts prepared by dry or wet impregnation are essentially the same. At low metal oxide contents the catalysts are of the monolayer type. At higher metal oxide contents also bulk compounds are present, which is demonstrated by means of X-ray diffraction as well as temperature-programmed reduction. The maximum concentration of monolayer-type compounds corresponds to approximately one transition metal atom per square nanometer of the carrier. A correlation could be established between reducibility of the oxidic monolayer-type catalysts and the activity for thiophene hydrodesulfurization. This correlation appears to be in good agreement with the one reported earlier for the analogous ¿-alumina-based catalysts. Butene hydrogenation activity of the sulfided MoO3/SiO2 and WO3/SiO2 monolayer-type catalysts also correlates with reducibility of the oxidic systems. Due to the presence of bulk compounds the turnover frequency in hydrodesulfurization as well as hydrogenation decreases at higher metal oxide contents.