The electrochemical behaviour of ammonia (NH4+/NH3) in sodium perchlorate at pH 9 has been investigated on electrochemically grown anodic iridium oxide film (AIROF) electrode. In base electrolyte at pH 9, the Ir(IV)/Ir(III) surface redox couple exhibits unusually large separation of the oxidation/reduction peaks (Delta E-p similar to 750 mV) due to the local pH changes within the oxide film. These local pH changes are induced by protons released/consumed during the Ir(IV)Ir(III) redox activity. As a consequence, the local pH within AIROF changed up to 10 units during potential cycling. The presence of ammonia at pH 9 prevents these local pH changes because NH4+/NH3 buffer the solution inside the electrode. As a consequence, Delta E-p of Ir(IV)/Ir(III) was reduced 100 mV. It has been shown further that ammonia oxidation is mediated by Ir(V), resulting in the appearance of anodic peaks in forward and back-ward scans. (C) 2009 Elsevier B.V. All rights reserved.