Improvement of gas-barrier properties of poly(ethylene terephthalate) (PET) by blending with an aromatic polyamide, either poly(m-xylylene adipamide) (MXD6) or a copolyamide based on MXD6 in which 12 mol% adipamide was replaced with isophthalamide (MXD6-12I), was studied. Incorporating a small amount of sodium 5-sulfoisophthalate into the PET matrix compatibilized the blends and reduced the polyamide domain size to 100-300 nm. Thermal analysis showed that the polyamides had a nucleating effect on PET crystallization, whereas crystallization of the polyamides was inhibited especially in compatibilized blends. Without orientation, blending with 10 wt% MXD6 or MXD6-12I reduced oxygen permeability of PET by a factor of about 0.8 (P/P-PET) when measured at 43% relative humidity (RH), in accordance with the Maxwell model prediction. However, after biaxial orientation, oxygen permeability of blends with 10 wt% MXD6 was reduced by 0.3 at 43% RH, and permeability of blends with 10 wt% MXD6-12I was reduced by 0.4. Even at 85% RH, oxygen permeability was reduced by 0.4 and 0.6 for blends with MXD6 and MXD6-12I, respectively. Blending with an aromatic polyamide was even more effective in reducing carbon dioxide permeability of oriented PET. Transformation of spherical polyamide domains into platelets of high aspect ratio increased tortuosity of the diffusion pathway. The platelet aspect ratio predicted by the Nielsen model was confirmed by atomic force microscopy. Higher aspect ratio of MXD6 domains was ascribed to a lower T-g compared to MXD6-12I. Similar reduction in oxygen permeability was achieved in bottle walls blown from PET blends with MXD6 or MXD6-12I. (c) 2005 Elsevier Ltd. All rights reserved.