The transport phenomena of salts across a liquid membrane containing a nonionic surfactant, polyoxyethylene p-nonylphenyl ether, as a carrier were investigated. The surfactant, complexing with metal cations selectively, functions as a monomeric carrier or a reversed micellar carrier of a self-assembly system depending on the concentrations of the carrier and the source phase salt. The fluxes of the hydrophobic salts were large in general, and potassium ion was selectively transported by the carrier containing 20 ethylene oxide units. The mole ratio of the salt to the carrier in the complex was 1 in the forms of both monomer and reversed micelle. The transport equation was derived from the complex formation, and its destruction rates and the diffusion resistance were found to be functions of the activities of the source phase salt and the carrier. The theoretical values agreed very well with the measured values. Both the reaction rate constant and the diffusion coefficient decreased significantly with the change of the carrier form from the monomer to the reversed micelle. Therefore, the increase of the flux with the increase of the concentrations of the source phase salt and the carrier was suppressed by the formation of the reversed micelle.