129Xe NMR spectroscopy is a valuable tool to investigate heterogeneous polymer materials. Different NMR parameters can give information about the structure of the material, e.g. the 129Xe chem. shift provides information about the chem. compn. of copolymers, the 129Xe line width about Tg, and the Xe self-diffusion coeff. about domain sizes and porosity. In the presentation esp. the last aspect will be emphasized. The effective diffusion coeff., which depends on the diffusion time, for xenon in several polymers is detd. by pulsed field gradient echo (PFGE) NMR spectroscopy. Our exptl. results show that in certain semi-cryst. (co)polymers the self-diffusion of Xe atoms is enhanced by the presence of the crystallites!.