Improvement in oxygen gas barrier properties of polyester/polyamide blends used in packaging industry is the main objective of the present study. For this purpose poly(ethylene terephthalate) (PET)/poly(m-xylene adipamide) (nylon-MXD6) (95/5 w/w) and poly(ethylene terephthalate-co-isophthalate) copolymer (PETI)/MXD6 (95/5 w/w) blends have been prepared with a PET copolymer which consists of 5 wt.% sodium sulfonated isophthalate (PET-co-5SIPA) as compatibilizer and a carboxyl-terminated polybutadiene (CTPB) as filler by using a co-rotating intermeshing twin screw extruder. The effects of chemical architecture and morphology on oxygen gas permeability and processability were analyzed by using a range of characterization techniques including differential scanning calorimetry (DSC) scanning electron microscopy (SEM), oxygen gas permeability analyzer, and a special computer controlled uniaxial stretching system that provides real-time measurement of true stress, true strain and birefringence. The morphological analysis revealed that PETco-5SIPA was an effective compatibilizer for both PET/MXD6 and PETI/MXD6 blends. DSC analysis and spectral-birefringence technique were used to understand the thermal and stress-induced crystallization behavior of the blends. Morphological analysis of the films after biaxial stretching indicated that the spherical nylon phase was converted to 75 nm thick disks during stretching (aspect ratio L/W = 6) that creates a tortuous pathway for oxygen ingress. Stretching enhanced the barrier properties of PET/MXD6 and PETI/MXD6 blends. (C) 2009 Elsevier Ltd. All rights reserved.