The transfer of Cd²⁺, Pb²⁺, Hg²⁺, Pd²⁺, and Cu²⁺ ions across polarized nitrobenzene/water (NB/W) interface assisted by 1,4,7,10,13,16-hexathiacyclooctadecane (HTCO) present in NB-phase has been studied by means of cyclic voltammetry. Voltammograms of Cd²⁺ and Pb²⁺ ions can be interpreted as being due to the reversible transfer of the ions assisted by the formation of 1:1 metal(M)-HTCO complex in NB-phase. The formal formation constants of [PbHTCO]²⁺ and [CdHTCO]²⁺ complexes in NB-phase have been determined to be 10^11.7±0.3 and 10^7.9±0.3 M^-1, respectively, using the formal potentials of non-assisted ion(M)-transfer at NB/W interface (Δ^W_NB φ^0′_M). The Δ^W_NB φ^0′_M’s of Cd²⁺ and Pb²⁺ ions and Zn²⁺ and Tl⁺ ions as well have been determined on the basis of the tetraphenylarsonium-tetraphenylborate assumption and of the polarographic reversible half-wave potentials of the ions in NB- and W-media reported by previous authors. Voltammograms of Hg²⁺ and Pd²⁺ ions have been explained by that 1:1 M-HTCO complex is formed and soluble in both NB- and W-phases. The formal potentials of the transfer of [HgHTCO]²⁺ and [PdHTCO]²⁺ complex ions at NB/W interface have been determined. Voltammograms of Cu²⁺ ion are of irreversible nature.