The influence of biaxial deformation on the development of thermal, optical and mechanical properties of PEN and PEN/PEI blends were investigated. The refractive indices in the normal direction of biaxially stretched films decreased with the areal expansion ratio, lambda(MD) x lambda(TD) The Wide angle X-ray results showed that this is caused by the orientation of naphthalene planes that become parallel to the film surface as the expansion ratio increases. Blending PEI with PEN at concentration up to 20% was also found to reduce the naphthalene plane orientation, thereby improving the deformation behavior of PEN by eliminating highly localized neck formation attributed to this orientation behavior. The WAXS pole figure studies further indicate that biaxially oriented PEN films show bimodal (＇cross-hatched＇) orientation of the chain axes one population in the MD I direction and another distinct population in MD2 direction. This is also reflected in the mechanical properties where moduli exhibit equal values in MD1 and MD2 directions but lower values in other directions in the film plane. When blended with PEI at 10%-20% concentrations, the bimodal orientation is eliminated and the equal biaxially stretched films was found to exhibit in-plane isotropy.