The ring electrode of a Au-Au rotating ring-disk electrode was used to monitor the concentrations of I- and I-2 during oscillations in current that occur during I- oxidation at the disk electrode. With the ring electrode poised at a potential sufficient to oxidize I-, the anodic current at the ring decreased, while the anodic current at the disk increased, as was expected for a typical shielding experiment. However, when the ring was poised at a potential sufficient to reduce I-2 (or I-3(-)) in a collection experiment, the cathodic current also decreased simultaneously with the increase in anodic current at the disk. These results demonstrate convincingly that the oscillating current involves the periodic formation and dissolution of an I-2 film.