In this study, we report here on the use of ac impedance spectroscopy to investigate intraelectrode processes occurring during oxygen reduction in a KOH half-cell. Investigated were novel composite electrode structures containing high surface area activated carbon fibers entrapped within a sinter-bonded matrix of 2 mu m diam stainless steel fibers. Systematic variations of the metal fiber to carbon fiber loading in the composite and also the electrode thickness and void volume fraction allowed mechanistic discrimination and development of a detailed equivalent circuit model. for the electrode. AC impedance was performed at various applied potentials and imposed current densities to determine the relative magnitude of the resistances distributed among kinetic, ohmic, and transport processes as a function of reaction rate. A conceptual model proposed earlier for the active reaction zone within the air electrode is discussed.