Abstract The relationship between the rates of transport of alkali metal cations through a bulk chloroform liquid membrane containing polynactin or dibenzo-18-crown-6 as neutral carrier and the rates of uptake and release of cation at the interfaces between aqueous phase and membrane phase were investigated. The fluxes of cations through the membranes and cation-distribution ratios between aqueous solution and membrane were strongly dependent on the anions present. The distribution ratio increased in the following order: Cl − < NO 3 − < SCN − < ClO 4 −, and the flux increased in the same order as the distribution ratio, except for the fluxes of KSCN and KClO 4 with polynactin. In the case of polynactin, the flux of KSCN was comparable to that of KClO 4 in spite of the fact that KSCN was less soluble in the membrane than was KClO 4. In order to clarify the cause of this apparently contradictory behavior, the apparent rate constants of uptake and release of potassium were determined independently using an equation derived from Fick's first law of diffusion. From the rates of uptake and release, it was suggested that the overall rate of cation transport through the membrane was dependent on the rate of release rather than that of uptake.