A room temperature ionic liquid (IL) composed of a quaternary alkylphosphonium (trihexyltetradecylphosphonium, P66614(+)) and tetrakis(pentafluorophenyl) borate anion (TB) was employed within a water| P66614TB (w|P66614TB or w|IL) biphasic system to evaluate cesium ion extraction in comparison to that with a traditional water|organic solvent (w|o) combination. Cs-137 is a major contributor to the radioactivity of spent nuclear fuel as it leaves the reactor, and its extraction efficiency is therefore of considerable importance. The extraction was facilitated by the ligand octyl(phenyl)-N, N'-diisobutylcarbamoylphosphine oxide (CMPO) used in TRans- Uranium EXtraction processes and investigated through well established liquid| liquid electrochemistry. This study gave access to the metal ion to ligand (1: n) stoichiometry and overall complexation constant, beta, of the interfacial complexation reaction which were determined to be 1: 3 and 1.6 x 10(11) at the w| P66614TB interface while the study at w|o elicited an n equal to 1 with beta equal to 86.5. Through a straightforward relationship, these complexation constant values were converted to distribution coefficients, da, with the ligand concentrations studied for comparison to other studies present in the literature; the w|o and w|IL systems gave delta(alpha) of 2 and 8.2 x 10(7), respectively, indicating a higher overall extraction efficiency for the latter. For the w|o system, the metal ion-ligand stoichiometries were confirmed through isotopic distribution analysis of mass spectra obtained by the direct injection of an emulsified water-organic solvent mixture into an electron spray ionization mass spectrometer. (c) 2014 Elsevier B.V. All rights reserved.