The oxygen reduction reaction (ORR) is fundamental to fuel cells and metal–air batteries. However, its practical value depends on a mechanistic detail that cannot be resolved by ordinary voltammetry: whether oxygen is reduced directly by four electrons to hydroxide, or by two electrons to hydroperoxide. This article presents an accessible rotating ring–disk electrode experiment that makes this branching process directly observable. Oxygen reduction is measured on a platinum disk in oxygen-saturated 1 M potassium hydroxide solution at five different rotation rates. At the same time, a concentric glassy-carbon ring collects and re-oxidises the hydroperoxide leaving the disk. The Levich and Koutecký–Levich analyses of the disk currents illustrate mass transport control and the extraction of kinetic information, while the potential-resolved hydroperoxide yield and the average number of transferred electrons are obtained from the ratio of ring and disk currents. The measurements reveal a predominantly four-electron pathway (n ≈ 3.4–3.9), alongside a parallel two-electron route whose contribution (approximately 10–30% hydroperoxide) increases towards the onset potential. The mean Tafel slope is approximately 83 mV per decade, which means that a tenfold increase in the magnitude of the kinetic current requires an overpotential change of approximately 83 mV within the fitted region. Its intermediate value between the commonly discussed ~60 and ~120 mV per decade regimes is consistent with coverage- and potential-dependent ORR kinetics on Pt, and it should not be assigned to a single elementary rate-determining step. Beyond the numbers, the experiment demonstrates why the ring is indispensable: it transforms an abstract concept into a measurable quantity that students can interpret from a plot. Furthermore, it distinguishes between reliable and unreliable observables (the ratio-based electron number and hydroperoxide yield versus absolute diffusion coefficients and electron numbers from slope analysis). An open-source analysis workflow lowers the barrier to adoption. This approach is suitable for advanced physical, analytical or inorganic chemistry courses, linking a key electroanalytical technique to a reaction of central importance in electrochemical energy conversion.

oxygen reduction reaction, rotating ring–disk electrode, hydrodynamic voltammetry, collection efficiency, Koutecký–Levich analysis, alkaline energy conversion