The transfer of alkylammonium cations across the water/2-nitrophenyl octyl ether (NPOE) interface was studied at the three-phase junctions supported by a carbon ink screen-printed electrode (SPE) using cyclic and square-wave voltammetry. A microliter droplet of NPOE containing 7,7',8,8'-tetracyanoquinodimethane (TCNQ) was immobilized on the surface of the SPE and was immersed in aqueous solutions containing various alkylammonium salts. A plot of the half-wave transfer potential versus the logarithm of the cation concentration exhibits a close to Nernst slope. Moreover, a plot of the half-wave transfer potentials against the formal transfer potentials of alkylammonium cations yielded a slope close to unity. All these experimental facts suggest that the reduction of TCNQ at the SPE/NPOE interface is accompanied by a simultaneous cation transfer across the NPOE/water interface to preserve the electroneutrality of the organic microdroplet, namely the overall process corresponds to a cation-coupled electron transfer reaction. Finally, this approach allowed measuring the formal transfer potential of highly lipophilic cation, such as bis(triphenylphosphoranylidene)ammonium. (C) 2010 Elsevier B.V. All rights reserved.