Hydrodesulfurization (HDS) of thiophene over silica-modified alumina-supported platinum (Pt/SiO₂-Al₂O₃) catalysts was examined. The HDS activity of a Pt/Al₂O₃ catalyst was enhanced by silica modification. However, the optimal silica loading for HDS activity could not be identified. The dispersion of platinum on alumina, as measured by the hydrogen adsorption method, was enhanced by silica modification and the optimal silica loading for platinum dispersion was 10 to 40 wt%. The acidity of SiO₂-Al₂O₃ was evaluated by 2-propanol dehydration (200°C) and cumene cracking (400°C). The optimal silica loading for the acidity of SiO₂-Al₂O₃ was 90 wt%. Furthermore, the presence of Brönsted acid sites on SiO₂-Al₂O₃ was confirmed by a pyridine adsorption method with FT-IR spectroscopy. The high HDS activity of the Pt/SiO₂-Al₂O₃ catalyst is caused by both high Pt dispersion and formation of Brönsted acid sites.