The behavior of the O 2 |HO 2 redox couple at equilibrium on a commercial uncatalyzed carbon-polytetrafluoroethylene (PTFE) oxygen-diffusion electrode, fed with O 2 partial pressures between 0.21 and 1.0 atm, has been studied by electrochemical impedance spectroscopy. Measurements have been made in the open-circuit potential using aqueous solutions with KOH concentrations in the range 1.0-6.0 mol dm 3 and HO 2 concentrations up 50 mmol dm 3 at 25.0°C. Under these conditions, the system is controlled by activation, the charge-transfer resistance being much higher than ohmic, adsorption, and diffusion resistive elements. The double-layer capacities show that the wetted electroactive areas, much smaller than the total area, depend on the KOH concentration used for activation. True exchange current densities of about I μA cm 2 are obtained, while their apparent values are two orders of magnitude greater, since the sluggish reaction is compensated by the high electroactive area. The cathodic process is a first-order reaction with respect to the O 2 feed, and Independent of HO 2 and OH concentrations. For the anodic one, a zero order for O 2 and a first order for HO 2 and OH are calculated. These results agree with the previously proposed mechanism for the O 2 reduction to HO 2 on the same electrode from voltammetric studies. Indirect evidence on a weak adsorption of HO 2 is found from the impedance diagrams.