The diffusion process of a solvent (dioxane) in samples of different cross-linked polystyrenes can be monitored by combined NMR imaging and solid-state NMR techniques. NMR imaging data reveal a strong dependence of the diffusion behavior upon the cross-link density : in the case of lowest cross-linking (1 %), a case II diffusion process takes place, whereas in the case of highest cross-linking (5 %), Fickian diffusion occurs. The 2.5 % cross-linked polystyrene shows intermediate behavior. Solid-state NMR relaxation parameters indicate great differences between the unswollen and the swollen state. Whereas the relaxation data obtained in the unswollen state are almost independent of the degree of cross-linking, great differences are observed between the 1 % and 5 % cross-linked polymers in the swollen state, with both the highest overall mobility and, in particular, a high mobility of the phenyl group in the case of the 1 % cross-linked material. This behavior can be directly related to the different types of diffusion, suggesting that the strong interaction of the highly mobile phenyl groups with the dioxane in the low cross-linked material favors case II diffusion. In the 5 % cross-linked materials, with a lower overall mobility and no predominant high phenyl group motion, Fickian diffusion occurs. Increased cross-linking corresponds to increasing T1rhoC and T1C values, thus indicating reduced mobility.