The permeability of block copolymer vesicles is studied using pulsed field gradient nuclear magnetic resonance spectroscopy together with a numerical data analysis procedure. Polyethylene oxide molecules of various molecular masses are used to sample the permeability of the vesicle membrane by observing the trans-membrane exchange process under equilibrium conditions. For shorter polyethylene oxide chains, the analysis yields a nearly linear dependence of the logarithmic trans-membrane exchange rate on the hydrodynamic radius of the sample molecules. (C) 2004 American Institute of Physics.