Attempts have been made to investigate the effects of ethylene-propylene-diene monomer (EPDM) rubber structural parameters on the developed microstructure, mechanical properties, rheology, and oxygen gas permeability of EPDM/organically modified montmorillonite (O-MMT) nanocomposite samples prepared via melt mixing. Maleic anhydride grafted EPDM (EPDM-g-MAH) has been employed as an interfacial compatibilizer. The influence of the EPDM melt viscosity and chain linearity on the extent of exfoliation of the clay nanolayers has been evaluated through the calculation of the nanolayer aspect ratio (length/thickness) with the Halpin-Tsai model. The results are consistent with the X-ray diffraction patterns of the samples. The flocculation of the clay nanolayers has been found to be more probable when O-MMT is mixed with highly branched, low-molecular-weight EPDM. More exfoliation occurs when EPDM rubber with a high molecular weight but low branching is used. This has been confirmed by more nonlinear melt rheology behavior and broadening of the retardation time spectra. Maleated EPDM has been shown to be effective in enhancing the molecular intercalation of the clay nanolayers and the prevention of flocculation in both low-molecular-weight and high-molecular-weight EPDM matrices. Dynamic melt rheology measurements have revealed nonterminal behavior within the low-frequency range by interfacially compatibilized molten samples with an EPDM-g-MAH/clay ratio of 3, regardless of the matrix molecular weight and chain linearity. (C) 2007 Wiley Periodicals, Inc. J Appl Polym Sci 106: 4209-4218, 2007