The permeability of vesicles of the synthetic surfactant sodium 4-(1′-heptylnonyl)benzenesulfonate changes when the crown ether ionophore dicyclohexano-18-crown-6 is added to the vesicle wall. The permeabilities of both potassium and sodium increase, with the potassium increase being much larger. The increase is not the result of defects introduced by the crown ether. Two structural models are proposed to account for the observed selective transport. The addition of crown ether also results in an increase in the concentration of small nonvesicular aggregates present in equilibrium with the surfactant vesicles, but does not change the vesicle size.