We present a novel synthetic method for the fabrication of encapsulated nanosized catalysts that have a core/shell structure consisting of numerous inorganic nanoparticles confined within the hollow core of a mesoporous carbon capsule. Specifically, we applied this synthetic strategy to CoFe2O4 nanoparticles (size similar to 10 nm), which were encapsulated within inner core of porous carbon capsules of ca. 350 nm diameter. We examined the catalytic activity of this core/shell catalytic composite toward H2O2 decomposition (Fenton reaction) and found that decomposition rates measured for this material were considerably higher than those for the unsupported nanoparticles and other nanocatalysts reported in the literature. The synthetic strategy described here could be extended to the preparation of other catalysts with similar core/shell structures. (C) 2007 Elsevier Inc. All rights reserved.