Membrane permeability of dispersed block copolymer vesicles is studied in the equil. state using pulsed field gradient NMR spectroscopy. It is found that the permeability for water mols. strongly depends on the ethanol content in the dispersion: an increase of the ethanol concn. from 0 to 40 vol% leads to a decrease of the av. residence time by a factor of two. The phenomenon is completely reversible as the subsequent redn. of the ethanol concn. leads to the original permeability characteristics. Hence, it can be used to reversibly control the permeability of vesicle membranes.