Digital simulation was performed to obtain a better understanding of the characteristics of the voltammogram for ion transfer from one aqueous solution, W1, to another, W2, through a membrane, M, which was obtained by scanning the potential difference between W1 and W2 and measuring the current between W1 and W2 under the condition that W1, M and W2 contain sufficient concentrations of supporting electrolytes. When one kind of objective ion, P+, was present in the membrane system in addition to supporting electrolyte ions, the number of current peaks observable in the voltammogram. depended on phase(s) where P+ was added. When P+ is added to W1, M, W1 and W2 or W1, M and W2, 1, 2, 2 or 3 pairs of positive and negative current peaks were observed. When P+ was added to W1 and an anion, X-, which transfers more easily than supporting electrolyte ions at the M \ W2 interface was added to W2, the peak potentials due to the transfer of P+ at the W1 \ M interface coupled with the transfer of X- at the M \ W2 interface and the peak separation between the positive and negative current peaks, DeltaE(p), depended on the ratio of concentrations of P+ and X-, even though the transfers of P+ and X- at the interfaces were reversible. The peak current was not proportional to the concentration of P+ unless the concentration of X- was much larger than that of P+. The reasons for the above-mentioned results were elucidated by considering complementary transfers of ions at the W1 \ M and M \ W2 interfaces. (C) 2003 Elsevier Science B.V. All rights reserved.