The transport of Cs+ across a bilayer lipid membrane between two aqueous phases containing chloride and per chlorate salts was readily observed when compared to using K+, because Cs+ is more hydrophobic than K+. After the Cs+ was distributed from the aqueous to the BLM with a counter anion, such as Cl+ and ClO4-, the antiport of Cs+ and the counter anion across the bilayer lipid membrane was caused by applying the potential difference between the two aqueous phases. By comparing the permeability in the presence of chloride salts with that in the presence of perchlorate salts, it can be appreciated that the standard potential for the transport of Cs+ from the aqueous phase to the bilayer lipid membrane is about 60 mV more negative than that of K+. According to the ion transport mechanism, the accumulation of Cs+ within living cells is assumed to be caused by the membrane potential, which is mainly generated by the transport of K+ across the cell membrane between the outside and inside. By use of a liquid membrane cell, it was verified that the membrane potential generated by the concentration ratio of K+ between two aqueous phases caused the accumulation of Cs+. (C) 2016 Elsevier B.V. All rights reserved.