Anodically formed iridium oxide film (AIROF) in trifluoromethanesulfonic acid (TFMSA) was used as a model system to examine the possible influence of Nafion on the electrochemical properties of iridium oxide catalysts for oxygen evolution reaction (OER) in polymer electrolyte membrane water electrolysis. A series of observations was made that is consistent with a significantly higher degree of adsorption of sulfate than triflate at the oxide surface. In TFMSA, the reaction rates for the OER are significantly higher than in H2SO4, and the flatband potential in TFMSA is higher than in sulfuric acid. In solutions of Fe(CN)(6)(3-/4-) and sulfuric acid, the reduction currents associated with the redox couple were much smaller than the oxidation currents. The same cathodic "blocking" was observed in TFMSA and Fe(CN)(6)(3-/4-). However, in the presence of small amounts of chloride ions (10 ppm) in TFMSA, the AIROF electrode showed no such cathodic blocking of the faradaic current and the electrode behaves similarly to a metal electrode. The chloride ions do not affect the redox behavior with sulfuric acid as the electrolyte. The absence of cathodic blocking in TFMSA with chloride ions is also suggested to be due to the lesser degree of electrolyte adsorption than in sulfuric acid; the oxide and substrate being more open to chloride hindering reoxidation of oxide cracks. (c) 2010 The Electrochemical Society. [DOI: 10.1149/1.3490671] All rights reserved.