The behaviour Of Co3O4 electrodes was investigated in alkaline solution during oxidative runs at different anodic potentials up to the value of oxygen evolution reaction (OER). Voltammetric results indicate the presence of several solid-state surface redox transitions (SSSRT) which lead to the formation of Co(IV) containing species which are the specific catalytic phases for OER. Electrochemical impedance measurements were done in different ranges of potential and frequency in order to better characterise the SSSRT and the capacitive response of the electrode/electrolyte interface. (c) 2007 Elsevier Ltd. All rights reserved.