The permeability of ethylene oxide-2-vinylpyridine diblock copolymer vesicles was studied using pulsed field gradient NMR spectroscopy. Polyethylene oxide mols. of various mol. masses were used as tracers for the observation of the trans-membrane exchange process under equil. and non-equil. conditions. For small polyethylene oxide chains (M w < 3400 g/mol), the anal. yielded a strong and roughly linear dependence of the logarithmic trans-membrane exchange rate on the hydrodynamic radius of the sample mols. With mol. 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.