Miscibility and morphology of poly(ethylene 2,6-naphthalate)/poly(pentamethylene terephthalate) /poly(ether imide) (PEN/PPT/PEI) blends were Studied by differential scanning calorimetry (DSC), optical microscopy (OM), proton nuclear magnetic resonance imaging (H-1-NMR), and wide-angle X-ray diffraction (WAXD). OM and DSC results from ternary blends revealed the immiscibility of PEN/PPT/PEI blends, but ternary blends of all compositions were phase-homogeneous following heat treatment at 300 degrees C for over 60 min. Annealing samples at 300 degrees C yielded an amorphous blend with a clear and single T-g at the final state. Experimental data from H-1-NMR revealed that PEN/PPT copolymers (ENPT) were formed by the so-called transesterification. The effect of transesterification on glass transition and crystallization was discussed in detail. The sequence structures of the copolyester were identified by triad analysis, which showed that the mean sequence lengths became shorter and the randomness increased with heating time. The results reveal that a random copolymer improved the miscibility of the ternary blends, in which, the length of the homo segments in the polymer chain decreased and the crystal formation was disturbed because of the irregularity of the structure, as the exchange reaction proceeded. (c) 2006 Wiley Periodicals, Inc. J Appl Polyrn Sci 100: 3840-3849,2006.