The facilitated transport mechanism of a metal cation across a supported liquid membrane was examined in the case of a Cd2+ flux mediated by the natural ionophore lasalocid; the polypropylene membrane was impregnated with o-nitrophenyl octyl ether. Impedance and membrane potential measurements were carried out along with flux measurements of the Cd2+/H+ counter transport. A proton flux was inferred from the membrane potential measurements performed under open circuit, zero electrical current, conditions. This proton flux was confirmed by impedance measurements performed under closed circuit, non-zero electrical current, conditions. The main consequence of this proton transfer under open circuit conditions (which are the normal membrane separation conditions) was the building of interfacial potentials. These potentials modify the ionic concentrations at the membrane interfaces, and thus render invalid the mathematical treatment of the cation flux based on the equilibrium ion exchange concept which is generally used. When the potential dependence of the effective interfacial reactions is incorporated into the transport model, the theory can account quantitatively for the experimental fluxes measured under open circuit conditions.