Mixtures of poly(ethylene terephthalate) and poly(ethylene naphthalene-2,6-dicarboxylate) have been transesterified at temperatures between 553 and 573 K. The partially transesterified polymers were analyzed using H-1 NMR, and the rate constants for transesterification were obtained using a kinetic expression based on a second-order reversible reaction mechanism. The hydroxyl end groups have been shown to have a significant effect on the kinetics of the reaction by comparing rate constants obtained for the same polyester mixture but with hydroxyl end groups quantitatively esterified by trifluoroacetic acid. Carboxyl end groups do not have such a significant role in transesterification, and from the variation of rate constant at one temperature for a series of PET polymers with differing carboxyl to hydroxyl end groups, it appears that transesterification by direct ester-ester interchange is very small. The NMR data have also been used to calculate the randomness factor and number-average sequence length of the terephthalate sequences in the copolymer as transesterification progresses. The initially immiscible polyester mixture becomes miscible after a critical amount of transesterification takes place; no matter what reaction temperature is used, this homogenization takes place when the randomness factor has a value of 0.5. This does not correspond to completely random copolymer formation but suggests that the number-average sequence length of the terephthalate units in the copolyester is ca. 4-8.