The electrochemical oxidation of ammonia (NH4+/NH3) in sodium perchlorate was investigated on IrO2 electrodes prepared by two techniques: the thermal decomposition of H2IrCl6 precursor and the anodic oxidation of metallic iridium. The electrochemical behaviour of Ir(IV)/Ir(111) surface redox couple differs between the electrodes indicating that on the anodic iridium oxide film (AIROF) both, the surface and the interior of the electrode are electrochemically active whereas on the thermally decomposed iridium oxide films (TDIROF), mainly the electrode surface participates in the electrochemical processes.