Structure evolution that took place during the constrained annealing of uniaxially constrained width oriented Poly (ethylene terephthalate) and Poly (ether imide) blend films was studied using real time spectral birefringence measurement technique and off line differential scanning calorimetry, wide and small angle X-ray scattering and mechanical testing. At lower deformations, heat setting generally lead to complete elimination of birefringence due to relaxation in blends containing as little as 10% PEI. At high deformation levels, annealing leads to initial relaxation nearly to optical isotropy followed by reversal and increase in birefringence associated with development of crystallization. The ultimate birefringence decreases with increasing PEI content as it disrupts crystallization. Latter behavior indicates that despite achieving near optical isotropy during initial relaxation phase, these films still contain oriented nuclei that facilitate the oriented crystallization of the PET phase at longer annealing times as evidenced by the off line WAXS and SAXS experiments. (C) 2016 Elsevier Ltd. All rights reserved.