The mechanism of alkali-metal transport across the bilayer wall of vesicles made from double-tailed synthetic surfactants has been determined. In aqueous solution, the alkali-metal cations sodium, potassium, and cesium pass through the bilayer of sodium 4-(1-heptylnonyl)benzenesulfonate and sodium 4-(1-nonylundecyl)benzenesulfonate as unhydrated, unassociated cations. Transport occurs through transient holes in the vesicle bilayer; surfactant monomers rearrange such that only head groups are exposed to the aqueous medium in the pore (so-called "inverted pores"). Other mechanisms such as passage of hydrated ions or solvation of ions or ion pairs in the membrane are not active. Measured activation energies for cation transport through pure bilayers are consistent with the proposed mechanism.