The mechanism for the electrodeposition of iridium onto glassy carbon and platinum substrates has been investigated. Iridium coatings were characterized by X-ray photoelectron microscopy to determine their chemical compositions and the morphologies of deposits were observed by scanning electron microscopy. The deposition of iridium on glassy carbon electrodes requires, in a first stage, the formation of hi surface sites. These sites, generated by reduction of Ir3+ ions for large overpotential, allow the adsorption of H atoms which act as the reducing agent for Ir3+ ions. In contrast, on platinum, Ir electrodeposition occurs readily due to a high Had, surface coverage. The optimal H Is surface coverage for the electrodeposition of Iron Pt is close to 0.5. Electrochemical quartz crystal microbalance measurements demonstrated that the Faradaic efficiency of Ir deposition on Pt is very low (0.2-1.7%). (C) 2011 Elsevier Ltd. All rights reserved.