The cyclic voltammetric behavior of Vulcan XC-72 / Nafion(R) and carbon-supported Pt / Nafion(R) interfaces was studied by using the powder microelectrode technique. It was concluded that catalyst particles not in direct contact with Nafion(R) also contributed to the cyclic voltammograms although the electrode /Nafion(R) interfaces formed only at the end face of the electrodes. The phenomena were interpreted in terms of surface conductivity and spillover of adsorbed species over the surface of catalyst particles. The effects of electrochemical pretreatment of the electrodes and the prolonged potential cycling on the cyclic voltammograms have been observed, which indicated that surface conductivity, and surface diffusion as well as spillover were sensitive to the surface status of the catalyst particles.