A theory of cyclic voltammetry of ion transfer across a liquid membrane has been presented based on the Nernst-Planck equation and the electroneutrality assumption. The initial conditions are given by the partition equilibrium of ions between the membrane and the two bathing solutions. Current-potential curves are calculated for the case of reversible transfer of Na+ across the membrane/solution boundary and the complete dissociation of electrolytes in the membrane, taking account of time-dependent solution resistance and the diffusion potential. The peaks appear only in the limited range of the scan rate at a given thickness of the membrane. The model explains wide peak separation which has been reported in the voltammetry of ion transfer in the presence of lipophilic ions. Upon imposing the voltage across the membrane, the phase-boundary potential at each side of the membrane varies with time and, hence, the ion partitioning at the membrane/bathing solution interface is a time-dependent process. (C) 1998 Elsevier Science Ltd. All rights reserved.