A method including cryogenic grinding, melt pressing from the molten state, and quenching was used to prepare blends of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalene 2,6-dicarboxylate) (PEN) in which the two phases were highly dispersed. The effect of melt-pressing times on the thermal properties and relaxation behavior of PET/PEN films were characterized with differential scanning calorimetry and dielectric spectroscopy. For short melt-pressing times, two glass-transition, two crystallization, and two melting peaks were observed, indicating the presence of PET-rich and PEN-rich phases in these blends. Longer melt-pressing times revealed a single glass transition and a single a-relaxation process, showing that PET-PEN block copolymers were likely to be formed during the melt pressing. The experimental findings were examined in terms of the transesterification reactions between the blend components, as revealed by H-1 NMR measurements.