The facilitated ion transport from one aqueous phase, W1, to another, W2, across a bilayer lipid membrane, BLM, containing valinomycin, Val, as an ionophore was investigated by voltammetry. Cyclic voltammograms for the ion transfer were symmetrical about the origin (0 V 0 A) and the magnitude of the ion transfer current increased with an increase in the absolute value of the applied potential. The magnitude of the ion transfer current at a definite potential in the voltammograms depended on the cation species added to W1 and W2 and was proportional to the concentration of Val in the BLM. The magnitude of the ion transfer current at a definite potential also varied in proportion to the hydrophobicity of the counter anion in W1 and W2. Taking into account the conjugated ion transfers at the W1\BLM and BLM\W2 interfaces, the positive current that flowed from W1 to W2 across the BLM was attributable to both the transfer of the complex-forming cation from W1 to the BLM and the transfer of the anion, which was distributed in the BLM as the counter ion from W2 to W1. The transfer from the BLM to W1 occurred at the W1\BLM interface and both the transfer of the cation from the BLM to W2 and the transfer of the anion from W2 to the BLM at the BLM\W2 interface. The negative current was then attributed to the opposite reaction. The voltammograms were asymmetrical with the origin when the ion components in W1 and W2 were different. (C) 2004 Elsevier B.V. All rights reserved.