The transfer of a cation across a supported liquid membrane is studied both theoretically and experimentally during a linear potential scan. The transferring cation was originally present both in the membrane and in the flanking aqueous phases, thus working as a potential-determining ion in the system. With this set-up, the transfer kinetics of hydrophobic ions which usually determine the limits of the potential window of an ideally polarised liquid / liquid interface can be studied, even if the standard potential of transfer is unknown. The probe cation studied was tetrabutylammonium, and the membrane phase was o-nitrophenyloctyl ether. The observed standard rate constant k(0) took a surprisingly low Value of ca. 2 x 10(-4) cm s(-1); however, this agrees with the Values measured with a rotating diffusion cell under steady-state conditions. It is therefore suggested that the different values obtained with different techniques reflect the structural changes of the liquid / liquid interface due to the electric current.