Pt-based alloy catalysts such as carbon black-supported Pt-Co catalysts (Pt-Co/CB) are used in the oxygen reduction reaction in polymer-electrolyte fuel cells (PEFCs) and are generally treated at high temperatures to allow for alloy formation. However, treatment of these catalysts at high temperatures results in serious aggregation of alloy particles. In this study, a Pt-Co/CB catalyst was covered with silica layers to inhibit the aggregation of alloy particles during treatment of the catalyst at high temperature for alloy formation. Coverage of Pt-Co/CB with silica promoted Pt-Co alloy formation with a high alloying degree and a small particle size, whereas the alloying degree of Pt-Co alloys in Pt-Co/CB without silica-coating was low, and these alloy particles were seriously aggregated during treatment at high temperature. These silica-coated Pt-Co/CB catalysts showed high activity and excellent durability for the oxygen reduction reaction. (C) 2010 Elsevier Inc. All rights reserved.