Processes taking place at the metallic nickel electrode in basic solutions at potentials negative of -500 mV vs. SCE were investigated. Two kinds of the electrodes were examined: freshly deposited and subjected to previous polarisation at potentials positive to -500 mV where the irreversible phase transition between various forms of Ni(II) compounds takes place ("aged" Ni electrode). Oxidation of fresh Ni electrodes is accompanied by a mass increase smaller than that expected for usually proposed products of Ni oxidation: Ni(OH)(2) and NiO. Adsorbed species, i.e., H2Oads/OHads- participate in generation of Ni(II) compounds. Obtained results indicate the existence of bulk hydrogen absorption in fresh Ni electrode at sufficiently negative potentials. This process is the prevailing one for "aged" Ni electrode below -500 mV vs. SCE. (c) 2005 Elsevier B.V. All rights reserved.