The transport of alkali metal cations by the carboxylic lonophores dianemycm, monensin, and nigericin was studied using a model membrane system in which a solution of the ionophore in a dense, water-immiscible solvent fills the bottom of a "U"-shaped glass tube, forming a liquid "membrane" between two aqueous salt solutions. Small samples of the salt solutions were taken over a period of several days and analyzed by flame spectroscopic techniques to measure the relative rates of exchange of two competing cations on one side of the membrane for a third cation on the opposite side. With ortho-dichlorobenzene as the membrane solvent, the "selectivity sequences" obtained from the relative transport rates were similar to published sequences resulting from equilibrium studies with one- and two-phase systems. However, they resemble more closely the sequences obtained from non-equilibrium experiments on biological and non-biological membrane systems. The transport of the alkali metal cations was strongly influenced by both the pH and the buffering capacity of the aqueous phases. Experiments with chloroform and carbon tetrachloride as the membrane solvents indicate that the transport rate is also effected by a change of solvent.