A mini gas diffusion electrode (M-GDE) which is accessible to gaseous reactants and electrolyte has been developed for studying fuel cell catalysts and reactions by simply depositing catalyst ink onto a gas diffusion substrate of around a millimeter in diameter. Analogous to traditional gas-diffusion electrodes. the M-GDE comprises a carbon-paper gas diffusion layer and a catalyst-ink derived catalyst layer but with considerably decreased geometric surface area. The M-GDE has been characterized using electro-chemical oxygen reduction reaction (orr) on carbon-supported platinum in sulfuric acid solution as a probe reaction. On the steady-state voltammograms. no diffusion-limiting currents are observed over a wide potential range of 0.7-0.95 V which is relevant to fuel cell operations. The absence of the diffusion limiting currents suggests that the interfacial oxygen mass transport is fast and the measured currents could be directly used for kinetic analysis without the need of mass transport correction. The Tafel plot exhibits two linear regions-a wide region with a slope of around 2RT/F followed by a narrow region with a slope of around 2 x 2RT/F at higher overpotentials. This Tafel behavior is similar to literature results for the orr on ordinary gas diffusion electrodes. The mass activities and specific activities of the carbon-supported platinum for the orr directly measured from the steady-state voltammograms are consistent with literature data measured under similar conditions. All these facts strongly indicate that the M-GDE is a promising technique for the investigations of fuel cell reactions and nanosized catalysts at gas-catalyst-electrolyte interfaces. It retains the advantages of gas diffusion electrodes and provides the convenience of a microelectrode. (C) 2011 Elsevier Ltd. All rights reserved.