Multiwalled carbon nanotube (CNT)-supported Pt nanoparticles (Pt/CNT) were covered with silica layers by successive hydrolysis of 3-aminopropyl-triethoxysilane and tetraethoxysilane on CNTs with Pt metal precursors, followed by reduction with hydrogen. The Pt/CNT covered with silica layers (SiO2/Pt/CNT) was used as a cathode catalyst for a proton exchange membrane fuel cell (PEMFC). The activity of SiO2/Pt/CNT catalyst for the oxygen reduction reaction in a single-cell PEMFC was similar to that of Pt/CNT, in spite of the uniform coverage of Pt with silica layers, indicating that the coverage of Pt/CNT with silica layers did not appreciably decrease the catalytic activity. In addition, SiO2/Pt/CNT electrocatalyst showed high stability during potential cycling from 0.05 to 1.20 V vs reversible hydrogen electrode in an aqueous H2SO4 electrolyte, whereas Pt/CNT significantly deactivated during the experiment. The structural change of Pt species in these electrocatalysts during potential cycling was investigated by transmission electron microscopy images and Pt L-III-edge X-ray absorption fine structure. The crystallite size of Pt metal in SiO2/Pt/CNT did not change appreciably during the potential cycling, while Pt metal crystallites in Pt/CNT seriously aggregated. Silica layers enveloping Pt metal particles in SiO2/Pt/CNT prevent the dissolution and redeposition of Pt metal particles as well as the agglomeration of Pt metal particles on the supports. (C) 2008 The Electrochemical Society.