A mesoporous platinum catalyst (H-I-Pt) containing 34 Angstrom diam pores was examined as a potential catalytic material for the electrochemical reduction of oxygen. The material was characterized in sulfuric acid at room temperature by voltammetry as both a microelectrode and as a gas diffusion electrode in a half-cell configuration. The material showed high activity toward the reduction of dissolved dioxygen molecules, although only about one quarter of the internal pore surface area appeared to be electrochemically accessible. Its exchange current density compared favorably to that of commercial platinum.