The cooperative ion-transfer, the facilitated ion-transfer of metal ion with several kinds of ligands, has been studied by a potential-scan cyclic voltammetry and a current-scan polarography at the water/l,2dichloroethane (DCE) interface using a system which involves copper( l,lO-phenanthroline (phen), and l,l,l-trifluoroacetyiacetone (HTAA). Although under the condition of [Cu2+] > [phen] in the aqueous phase no wave appears, a new wave has been observed by dissolving HTAA in the organic phase. This wave shows quasi-reversible characteristics and the reversibility increases with an increase in [HTAA]. From the analysis of the voltammetric peak ratio, it has been concluded that the cooperative ion-transfer follows the slow formation of Cu(phen)TAA+ at the interface and that the species transferred is also Cu(phen)TAA+. Furthermore, at [HTAA] > 1.0 x 10m3M, a second anodic wave begins to appear at a higher potential than the first wave, without any corresponding cathodic wave, and this ion-transfer may be followed by very irreversible chemical reactions including interfacial adsorption. Comparing the results of cyclic voltammography and polarography, some interesting phenomena have been also pointed out and mechanisms of the quasi-reversible cooperative ion-transfer have been elucidated in detail.