A mosaic electrode array of TiO2 and Au disks in an ideally ordered arrangement with a uniform gap of 60 nm was fabricated using an anodic porous alumina substrate and used to confirm whether superoxide ions (O-2(-)) produced on TiO2 under UV irradiation diffused to the solution or adsorbed on the TiO2 surface until they were decomposed by disproportionation. The oxidation current of the Au electrodes was found to be suppressed by adding superoxide dismutase and catalase to the reaction solution, indicating that O-2(-) and hydrogen peroxide were formed by irradiation of UV light on the TiO2 surface and a small amount of O-2(-) diffused from the TiO2 surface to the solution. (C) 2004 The Electrochemical Society.