To better understand the reactivation of nickel cathodes by iron, the study was performed on iron membranes in 25% KOH at 80 degrees C. Membranes were cathodically treated and anodically polarised, and simultaneously hydrogen permeation was measured with the electrochemical technique. During cathodic potential sweeps, three peaks of current and of hydrogen permeation occurred. The peaks at about -0.80 V and -1.30 V vs. SHE were ascribed to reduction of Fe3O4 to HFeO2- and H+, and of HFeO2- to Fe, respectively. Electric charge of anodic oxidation of cathodically pretreated membranes was by over two orders of magnitude higher than the charge of desorbing hydrogen, and it increased with the pretreatment time. This was ascribed mainly to the oxidation of iron and its corrosion products. It was proposed that the reactivating effect of iron on Ni cathodes can be associated with the enhanced reactivity of iron freshly deposited by reduction of HFeO2-. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.