The stoichiometry of the Ir oxide reaction in sulfuric acidic solutions was investigated by tracking the pH dependence of its principal Ir( + III) Ir( + IV) redox peaks. as well as monitoring the film mass change with in-situ quartz crystal microbalance measurements. A pH dependence of ca. - 68 mV versus a pH independent reference electrode was found for films grown by potential pulsing using a wire electrode. indicating the injection:expulsion of protons and some solution anions during film reduction oxidation. The unaccounted mass change was interpreted as being due to water flux in/out of the film. The pH dependence. and hence, the ion stoichiometry appears to be independent of the acidic solution used to grow the Ir oxide film, while the acidic solution in which the oxide is cycled between its reduced and oxidized states appears to influence the Ir( + III) Ir( + IV) reaction stoichiometry. Crown copyright