We examined the partition of two amines, procaine and metoprolol, in ionic and charge-neutral forms between organic solvent (O) 1,2-dichloroethane and water (W) phases, each of which contained a common ion, [(CnF2n+1SO2)(2)N](-), as tetrabutylammonium and Li+ salts, respectively, to control the potential difference at the O vertical bar W interface, Delta(W)(O)phi. The interfacial system would be useful in the study of the partitioning of ionic species, which depends on Delta(W)(O)phi. With different pH levels, the dependence of the distribution ratio on Delta(W)(O)phi satisfied the prediction from the theoretical equation, giving the formal potentials for the transfer of the ionic forms at the O vertical bar W interface. We also examined the partition of nourseothricin, a mixture of streptothricin antibiotics with different sizes of lysine oligomers. They would exist as di-, tri-, tetra- and pentavalent cations in fully protonated forms. The distribution ratio for each ionic species could be determined simultaneously using an HPLC technique. The dependence of the distribution ratio on Delta(W)(O)phi for each species also satisfied the prediction, giving their formal potentials. Therefore, the present interfacial system can be applied when both ionic and charge neutral species are partitioned and when the sample is of low purity or is a mixture.