Poly(ethylene terephthalate)/organo-modified-montmorillonite (o-MMT) nanocomposites were prepared via melt compounding. A polyester ionomer was used as a compatibilizer to increase the interaction between the nanoclay and PET The nominal o-MMT content was 2 wt.% and the ionomer/organoclay (mass ratio) was 3: I. The samples were characterized by WAXD, SEM, TEM, TGA, rheometry, DSC, O(2) permeation and tensile testing. It was found that the addition of the ionomer improved the dispersion of the nanoclay particles in the PET matrix, leading to an exfoliated structure for the samples prepared by twin screw extrusion and by an internal mixer (Brabender). This was confirmed by larger complex viscosity and storage modulus at low frequency for molten samples. However, a subsequent processing using single screw extrusion to produce films resulted in thermal degradation of the organo-modifier of the clay and collapse of the gallery spacing. DSC results revealed that the cold crystallization temperature of nanocomposites-based films decreased and the melt crystallization temperature increased with the introduction of the organoclay, due to the strong heterogeneous nucleation effect of the clay particles. The tensile modulus of extruded films increased, while the yield strength remained constant with the incorporation of the organoclay. The oxygen permeability of PET-ionomer nanocomposites decreased as compared with samples containing no ionomer.