Voltammetry for ion transfer at the interface of two immiscible electrolyte solutions was demonstrated to be feasible at the interface between various mixtures of formamide (FA) and water (W) and 2,6-dimethylnitrobenze. Standard Gibbs transfer energies Delta G(tr)(0) of ions from W to FA + W were evaluated from the differences in the half-wave potentials of the voltammograms. Values of Delta G(tr)(0) for ions of moderately small volumes, such as (CH3N+, BF4- and ClO4-, were almost independent of the FA content, which was explained by the electrostatic and non-electrostatic interactions between the ion and solvents. Values of Delta G(tr)(0) for alkali metal ions decreased and those for halide ions increased with the increasing FA content, and the changes were remarkable in the range below 30% FA, which was explained in terms of short-range interactions between the ions and FA or W. The decrease in Delta G(tr)(0) for an unchanged moiety such as CH2 on addition of FA to W was interpreted in terms of the decrease in the cavity formation energy.