Reactively blended poly(ethylene terephthalate)-poly(ethylene naphthalate) films of different compositions and degrees of ester exchange reaction have been investigated by dielectric relaxation spectroscopy. The alpha -relaxation peaks associated with the glass transition have been fitted by the semi-empirical Havriliak-Negami relaxation function to frequency scans. The parameter relating to relaxation broadness, beta, and the relaxation strength, Delta epsilon, were quantified and were found to be a parameter sensitive to processing conditions. Both values were affected on a molecular level by concentration fluctuation and the molecular chain architecture (such as polymer chain blockiness). Molecular coupling of the blends was determined from the slope of log frequency maxima vs. reduced temperature T-g/T, as commonly done in coupling theory analysis. It was found that the initial materials, PET and PEN show very similar coupling behaviour. Molecular coupling of the blends was little affected by the blend composition or the degree of transesterification. Activation energies of molecular motions have also been determined and show a positive deviation from the rule-of-mixtures averages of the homopolymers which indicates greater chain hindrance to motion of both blocky or the random copolymers formed by the transesterification process.