The permeability of block copolymer vesicles was studied using pulsed field gradient nuclear magnetic resonance spectroscopy. Polyethylene oxide molecules of various molecular masses were used as tracers for the observation of the trans-membrane exchange process under equilibrium and non-equilibrium conditions. For small polyethylene oxide chains (M-w < 3400 g/mol), the analysis yielded a strong and roughly linear dependence of the logarithmic trans-membrane exchange rate on the hydrodynamic radius of the sample molecules. With molecular masses increasing above M-w = 3400 g/mol, the dependence of the exchange rate vs. the hydrodynamic radius decreased drastically, indicating two independent mechanisms of permeation. (c) 2005 Elsevier B.V. All rights reserved.