Nanocomposite vulcanizates based on ethylene-propylene-diene monomer rubber (EPDM) and organically modified montmorillonite with improved mechanical and barrier properties were prepared via a melt-mixing process in the presence of maleic anhydride grafted ethylene-propylene-diene monomer rubber (EPDM-g-MAH) as an interfacial compatibilizer. The effects of the EPDM Mooney viscosity as the matrix and also the compatibilizer molecular weight and its maleation degree on the developed microstructure were also studied. The annealing of the vulcanized nanocomposites based on a low-Mooney-viscosity EPDM matrix and low-Mooney-viscosity EPDM-g-MAH enhanced the flocculation of the dispersed clay platelets; this implied that the flocculated structure for the clay nanolayers was more thermodynamically preferred in these nanocomposites. This was verified by the decrease in the oxygen permeability of the nanocomposite vulcanizates with increasing annealing time. The tendency of the clay nanosilicate layers to flocculate within the matrix of EPDM was found to be influenced by the clay volume fraction, the maleation degree, and also, the Mooney viscosity of the compatibilizer. Interfacially compatibilized nanocomposites based on high-molecular-weight EPDM exhibited a more disordered dispersion of the clay nanolayers, with a broadened relaxation time spectra; this was attributed to the higher shearing subjected to the mix during the melt-blending process. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 120: 3133-3140, 2011