The electrode reaction Cu(II)/Cu(Hg) in complex acetate (Ac-) solutions has been studied at the equilibrium electrode potential by a cyclic current-step method. The data refer to an ionic strength of 1 M with sodium perchlorate as the supporting electrolyte and to a temperature of 25-degrees-C. Double-layer data were obtained from electrocapillary measurements. The kinetics indicates that the electron transfer Cu(II)/Cu(I) via the amalgam, which is the slowest step in the overall charge transfer, is substantially accelerated at increasing ligand concentrations within the range covered ([Ac-] greater-than-or-equal-to 20 mM). The results obtained show that Cu2+, CuAc+, CuAc2 and Cu2Ac3+ take part as oxidants. For ligand numbers nBAR > 0.2, CuAc+ and Cu2Ac3+ are predominant. The rate enhancement can primarily be explained by the formation of an electron-mediating carboxylate bridge between Cu(II) and the amalgam. Both Cu2Ac3+ and the corresponding reductant, the mixed-valence complex Cu(II)Cu(I)Ac3, can form such bridges, which probably is the cause of the very strong electrocatalysis displayed by these dinuclear complexes.