Immobilization of EPDM chains on the surface of carbon black and network structure in the rubber matrix of filled EPDM rubbers were studied by low-field proton NMR experiments. Advanced NMR experiments unambiguously show strong immobilization of EPDM chain fragments on the surface of carbon black. The thickness of the immobilized EPDM–carbon black interfacial layer is estimated to be ≥0.6 nm. The average number of monomer units per adsorption site is approximately nine, which suggests preferential chain adsorption at the crystal boundaries of carbon-black particles. The adsorbed chain fragments form physical (adsorption) junctions restricting chain mobility in the rubbery matrix outside of the interface. The cross-link density in filled EPDM is determined as a function of the filler type and its amount. The contribution of adsorption junctions to the total cross-link density is moderate as compared to the density of chemical cross-links and entanglement density. The mechanically effective network density in carbon-black-filled vulcanizates is determined by analysis of the stress–strain curves on the basis of the dynamic flocculation model. Comparison of the network density as measured by NMR and mechanical experiments shows significant differences which helps in better understanding of the reinforcement mechanism of filled rubbers. The study demonstrates that a relatively small amount of strongly adsorbed chains impacts the stress–strain properties of filled elastomers significantly.