The electrochemical behavior of Ir(100) electrodes has been characterized in various aqueous solutions. In general, this electrode behaves very similarly to the other single crystalline ones of the platinum group. As to special features, in perchloric acid solution the potential of the zero total charge was found to be within a sharp peak due to adsorption/desorption of hydrogen and anions; in the double-layer region there are indications of perchlorate reduction. In the accessible potential range no capacitance peak like that of Pt(111) was observed, and no Gouy-Chapman minimum could be identified. (c) 2006 Elsevier B.V. All rights reserved.