This work unveils physico-chemical mechanisms of highly selective extraction of Co2+-ions from their mixtures with Ni2+-ions using Mo6S8 (Chevrel phase) electrochemical transfer junction (ETJ). ETJ separates the feed and recovery compartments of the cell, in which electrolysis using different current densities is carried out. By parallel cyclic voltammetry, chronopotentiometry and electrochemical impedance studies in 3 electrode cells it was found that the potential of Co2+-ions extraction from the Mo6S8. electrode is by 0.1 V more negative, and its interfacial ion-transfer kinetics is more facile as compared to that for Ni2+-ions. This allows the surface of the ETJ, facing the recovery compartment of the cell, to operate as a highly selective ion gate: Co2+-ions are able to readily cross the ETJ/recovery solution interface, while Ni2+ ions are effectively blocked by this interface since the potential of these ions extraction is lower than that for Ni2+-ions from Mo6S8 electrode. The results obtained are important for emerging new technologies for separation/recovery of hazardous metal ions from leached waste solutions of spent rechargeable batteries. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.031303jes] All rights reserved.