Membrane permeability of dispersed block copolymer vesicles is studied in the equilibrium state using pulsed field gradient nuclear magnetic resonance spectroscopy. It is found that the permeability for water molecules strongly depends on the ethanol content in the dispersion: an increase of the ethanol concentration from 0 to 40 vol% leads to a decrease of the average residence time by a factor of two. The phenomenon is completely reversible as the subsequent reduction of the ethanol concentration leads to the original permeability characteristics. Hence, it can be used to reversibly control the permeability of vesicle membranes. (c) 2006 Elsevier B.V. All rights reserved.