Ключевые слова: кобальт, катодное восстановление, промежуточные частицы Cathodic reduction of iron-group metals, cobalt in particular, is usually accompanied by hydrogen evolution, which is thought to proceed as a parallel independent process by Volmer-Tafel or Volmer-Heyrovsky mechanism on the most active sites of the metal surface [1-3]. <...> Cobalt current efficiency can reach 95-98 % [4-5], but never equals to 100 %, even when deposited from metal-concentrated and pH-neutral electrolytes. <...> Current efficiency – current density (Dk Vladimir L. Krasikov1 THE ROLE OF ELECTROCHEMICAL COBALT REDUCTION INTERMEDIATES IN THE FORMATION OF OXYGEN-CONTAINING ADMIXTURES JSC Russian Institute of Radionavigation and Time, pl. Rastrelli, 2, St. Petersburg, 191124, Russia, e-mail: kras257@km.ru The article deals with the cobalt ions cathode reduction regularities occurred under various conditions. <...> It is shown, that if Co(II) ions reduction proceeds in two steps under relatively low current density, forming polycrystalline deposit, there may occur special conditions at the boundary of growing crystals that may lead to delay of the second electron transfer to Co(I) intermediate particle. <...> It results in its chemical or electrochemical oxidation formation of oxygen-containing compounds, incorporated into cathodic coating, instead of its reduction to the metal adatom. <...> The adsorbed Co(I) intermediates may chemically interact with surface-active substances and other components of electrolyte, as well as result the hydrogen formation by means of an unconventional mechanism which is different from Volmer-Tafel and Volmer-Heyrovsky mechanisms. <...> Keywords: cobalt, electrochemical deposition, intermediates. electrode layer is neglectable, which was confirmed by experiment with micro glass electrode [29-31] and cannot lead to insoluble compounds formation. <...> Under these conditions the only side reaction is water molecule discharge and hydrogen evolution that can be partially adsorbed by the metal as an admixture. <...> Current efficiency lowering, when Dk (0.1-1 A/dm2 limitations and increasing relative rate of water molecule discharge that can lead to cathode area alkalization and with the formation of insoluble metal hydroxide compounds included in cathodic coating [10-28]. <...> Metal current efficiency lowering, when Dk ), can be explained on the basis <...>