Impedance data were collected for nanostructured iridium oxide (NIROF) at potentials below those at which the oxygen evolution reaction commences. The measurements included thin oxide films covered by a protective Nafion (TM) layer and thicker composite Nafion (TM)-oxide electrodes. The time constants for the low-frequency diffusion process were approximately the same for both types of electrodes, indicating diffusion in individual particles in the porous electrode rather than across the film. The diffusion process involves trapping of the diffusion species. The impedance data indicated that there were no significant variations in conductivity of the oxides with potential, as opposed to what appears to be the case for anodically formed iridium oxide films (AIROF). This is interpreted to reflect differences in electronic structure between NIROF and AIROF. (C) 2009 Elsevier Ltd. All rights reserved.