Abstract QCMB data were obtained for Ir oxide films formed by standard potential cycling methods on an Ir sputter-coated quartz crystal. In sulfuric acid solutions, the mass change of the electrode in a single potential sweep was found to vary linearly with the oxide film charge density, obtained under equilibrium conditions, indicating that no artifacts due to a significant degree of flexibility of the surface film were being introduced in this solution. In sulfuric acid solutions, the mass of the oxide film increased/decreased by ca. 6 g per mole of Ir sites being reduced/oxidized. This is consistent with the injection of 1.5 H + and 0.25 SO 2− 4 per electron during reduction, as predicted from the unusual ca. 90 mV pH dependence usually observed, and the concurrent loss of 1 H 2O. In KOH solutions, the mass change is smaller and is in the opposite direction from that in acidic media. The oxide film loses ca. 3–4 g per mole of Ir sites during reduction, which would be consistent with the expulsion of OH − and a small amount of K +, together with the gain of one H 2O during the negative step. These QCMB results confirm that hydrous Ir oxide films exhibit many of the characteristics of a number of polymer-modified electrodes, and provide support for the involvement of solvent and the injection/expulsion of counter ions in addition to H +/OH − during the oxidation/reduction of Ir oxide films.