Pt inclusions in high-specific-area graphite (HS300, from Lonza) and Vulcan (XC-72R, from Cabot) have been syn thesized by the reduction of PtClx salts intercalated in these hosts. These materials perform well as catalysts for the reduction of oxygen in polymer-electrolyte fuel cells (PEFCs). After an activation time of 48 h during which the cathode is held at 0.5 V vs, the reference hydrogen electrode, it is found that catalysts based on Pt inclusions and commercial catalysts of the same Pt content and loading have similar mass and specific activities. However, in the same conditions, catalysts based on Pt inclusions perform better in the high-current region than the commercial catalysts. It has been shown by X-ray photoelectron spectroscopy analysis that, at the beginning of the activation time, a fraction of the Pt oxides and/or hydroxides present in the materials is reduced to Pt metal. This reduction improves the catalytic activities of the materials prepared in this study and also those of commercial catalysts. Further improvement is probably due to hydration of the catalyst layer and/or the washing out of insidious contaminants (e.g, chloride ions). Because higher current densities can be obtained with Pt included in graphite catalyst at a loading of 0.13 mg Pt/cm(2) than with commercial Pt supported on Vulcan catalyst at a loading of 0.15 mg Pt/cm(2), the use of cathode catalysts containing Pt inclusions is therefore a way of reducing the Pt loading at the cathode of PEFCs. However a limitation of the new materials is the fact that the maximum amount of Pt that can be included in Vulcan is only about 5 wt %, whereas it is only 10 wt % in graphite.