The mol. exchange of tracer mols. through the membranes of dispersed vesicles of the block copolymer poly(2-vinylpyridine-block-ethylene oxide) was studied by using NMR spectroscopy combined with pulsed field gradients. The hydrodynamic radius of the tracer mols. was varied systematically to obtain a permeability profile of the vesicle membrane. In addn., the effect of system parameters, such as temp., pH value, vesicle size, and thickness of the vesicle membrane, was studied. In the case of rapid exchange with av. residence times significantly smaller than 10 s, the permeation is obsd. under equil. conditions and the data are analyzed by using a simple anal. approach. For slow exchange processes with av. residence times above 10 s, the permeation is monitored in a time-resolved measurement under nonequil. conditions. Generally, the transmembrane exchange rate of the tracer clearly depends on its hydrodynamic radius. The characteristics of this dependence indicate the presence of two different mechanisms of membrane penetration, one dominating for smaller and one for larger tracer mols., resp. The exchange rate also shows a significant dependence on the bilayer thickness and on the vesicle diam. By contrast, no variation of the membrane permeability with the temp. or the pH value could be detected as long as the vesicles remain stable.