Multi-walled carbon nanotube-supported Pt metal particle (Pt/CNT) catalysts were covered with silica layers with different thickness and density. In the present study, the Pt/CNT covered with silica layers (SiO2/Pt/CNT) was prepared by the successive hydrolysis of 3-aminopropyl-triethoxysilane (APTS) and tetraethoxysilane (TEOS). The thickness of silica layers in SiO2/Pt/CNT could be controlled by varying the concentration of silica sources during the preparation of SiO2/Pt/CNT. The density of silica layers in SiO2/Pt/CNT was changed by varying the treatment temperature for the catalysts as well as by adjusting the pH for the hydrolysis of silica sources during the preparation of the catalysts. The catalytic performance of these SiO2/Pt/CNT as cathode catalysts in polymer electrolyte fuel cells was examined. Pt/CNT covered with silica layers of adequate thickness (ca. 6 nm) and high density had a high electrochemically active surface area as well as excellent durability. Dense silica layers in SiO2/Pt/CNT prevented the diffusion of cationic Pt species dissolved from Pt metal particles out of the catalysts. (C) 2009 Elsevier B.V. All rights reserved,