Novel information oil filler-elastomer interactions is obtained by combining solid-state H-1 low-Field NMR spectroscopy and equilibrium swelling experiments. Multiple-quantum (MQ) NMR experiments provide detailed quantitative molecular information oil the cross-link density of the elastomer matrix in it variety of Filled systems, indicating generally weak filler effects oil the overall cross-link density and oil the network homogeneity. Swelling experiments, as well as mechanical data, are additionally influenced by the matrix-filler and filler-filler interactions. Our approach is based oil comparing cross-link densities from NMR and (Flory-Rehner) swelling experiments, for which a masterline is always found in Unfilled elastomers. In filled elastomers two different scenarios are observed. If there are no interactions between the polymer chains and the filler Surface. no deviations from the masterline are detected because the swelling capacity of the composite is governed by the bulk polymer. Deviations from the masterline (reduced swelling) are exhibited by those composites that have strong rubber-filler interactions. In these cases, some fraction of the polymer is connected to the filler surface. which thus behaves like a giant cross-link, and the overall degree of swelling is thus reduced as compared to the bulk polymer. The novel experimental approach was used to evaluate filler-elastomer interactions in different composites and nanocomposites.