The theory of the cyclic and convolution potential sweep voltammetry of the reversible ion transfer across a liquid membrane is developed. The model system consists of the planar layer of an organic solvent separating two aqueous phases, each containing a supporting electrolyte. The transferred ion is supplied from one of the aqueous phases. It is shown that the coupling between the ion transfer processes at the two membrane interfaces has an effect on the shape and position of the voltammogram on the potential scale. The theory is used to evaluate the diffusion coefficient and the standard ion transfer potential of a semi-hydrophobic ion, i.e. protonated tetracaine, from voltammetric measurements in a supported o-nitrophenyl octyl ether (o-NPOE) membrane cell. These results agree well with the data inferred from measurements of the ion transfer across a single water \ o-NPOE interface.