Ion-transfer reactions at the interface between fluorous solvent 1,14,2,3,4,4,5,5,5-decafluoropentane (DFP) and water (W) were investigated voltammetrically. Within the potential window at the DFP vertical bar W interface, various non-fluorinated ions, including tetraphenylarsonium and tetraphenylborate, and fluorinated ions, including fluorocomplex and perfluoro-n-alkanoate ions, gave reversible voltammetric waves for their ion transfer across the DFP vertical bar W interface. Based on the tetraphenylarsonium-tetraphenylborate assumption, the formal potentials of their ion transfer could be determined in terms of the Galvani potential difference. Also, formal potentials for the transfer of bis(perfluoroalkanesulfonyl)imide ions at the DFP vertical bar W interface were determined potentiometrically. The formal Gibbs energies for the transfer of ions from DFP to W, which would provide the criteria necessary for evaluating the fluorophilicity/hydrophilicity of ions were calculated from the formal potentials, and were compared with the transfer energies from 1,2-dichloroethane to W and those from the organic solvent to DFP, as quantitative scales of the ions' lipophilicity/hydrophilicity and fluorophilicity/lipophilicity.