We investigated the structure of the barrier layers of nanoporous alumina formed by anodization of aluminum films laid over platinum electrodes by cyclic voltammetry technique and scanning electron microscopy. Two methods of anodization were employed in this study-the conventional sub-surface anodization method and a novel pipette anodization method. Measurements of the electron transfer rate constant values at these alumina-modified electrodes demonstrated very different alumina microstructures at the barrier layer region for these two types of alumina-modified electrodes. The sub-surface anodized electrode consists of a barrier layer, which forms a kinetic barrier for the electrochemical reaction of a redox probe at the underlying platinum electrode. In contrast, the quasi-reversible electrochemical behaviour of a redox probe at the pipette-anodized electrodes closely resembles the reaction observed at a bare platinum electrode. Scanning electron microscopy revealed pore structures along the underside surface of the alumina overlayer of a pipette-anodized electrode. These results indicate alumina modified eletrodes fabricated using the pipette anodization method comprise channels which penetrate the barrier layer, giving rise to 'barrier layer-free' alumina films. (c) 2006 Elsevier Ltd. All rights reserved.