H(1) mesoporous platinum surfaces formed by electrodeposition from lyotropic liquid crystalline templates have high electroactive surface areas (up to 60 m(2) g(-1)) provided by the concave surface within their narrow (≈2 nm diameter) pores. In this respect, they are fundamentally different from the flat surfaces of ordinary Pt electrodes or the convex surfaces of high-surface-area Pt nanoparticles. Cyclic voltammetry of H(1) mesoporous Pt films in acid solution is identical to that for polycrystalline Pt, suggesting that the surfaces of the pores are made up of low-index Pt faces. In contrast, CO stripping voltammetry on H(1) mesoporous Pt is significantly different from the corresponding voltammetry on polycrystalline Pt and shows a clear prewave for CO oxidation and the oxidation CO at lower overpotential. These differences in CO stripping are related to the presence of trough sites where the low-index Pt faces that make up the concave surface of the pore walls meet.