Compatibilized blends of poly(ethylene terephthalate (PET) with an aromatic polyamide such as poly(m-xylylene adipamide) (MXD6) have good transparency (T) because the constituent refractive indices (RIs) match closely. However, haziness is observed when the blends are stretched. This study demonstrated that stretching imparted a greater RI anisotropy to PET than to the aromatic polyamide. The resulting RI mismatch was responsible for the loss in T. Analysis of the strain-dependent birefringence revealed that different molecular deformation models described the intrinsic birefringence of the PET and aromatic polyamides. Hydrogen bonding of the polyamide may have been responsible for the difference. On the basis of these results, three approaches for improving T of stretched PET blends were attempted. Blends with a lower molecular weight MXD6 exhibited slightly higher T after stretching; however, they did not compare with stretched PET. Increasing the amount of compatibilizer reduced the particle size; however, the dimension of even the smallest particles exceeded the quarter wavelength after biaxial stretching transformed the spherical particles into platelets. Copolyamides based on MXD6 that incorporated isophthalate were designed to increase the RI of the polyamide and thereby reduce the RI mismatch with stretched PET. Unexpectedly, the poor T of stretched copolyamide blends was attributed to the high glass-transition temperature of the copolyamide, which hampered the molecular orientation. (c) 2005 Wiley Periodicals, Inc.