Blends of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) have been analyzed in terms of their crystal structures and morphologies. Wide angle X-ray diffraction (WAXS), small angle X-ray diffraction (SAXS), and density measurements were performed on blend samples crystallized quiescently and on samples strain induced crystallized. For blends quiescently crystallized isothermally, the crystallite dimensions calculated from the WAXS diffraction patterns via the Scherrer equation, along with the results from the SAXS, led to the conclusion that the blends crystallized independently. This means the major component of the blend crystallized, while the minor component did not crystallize and was forced into the interlamellar amorphous region. WAXS studies on the strain induced crystallized samples showed a change in the lattice parameters with composition, leading to the conclusion of co-crystallization for these blends. Density measurement showed a decrease in density with increasing PEN content indicating a reduction of the percent crystallinity of the blends as compared to the unblended homopolymers.