Small-angle neutron scattering has been used to determine the rate constants for transesterification reactions between hydrogenous and deuterated poly(ethylene terephthalate)s. Intimate mixtures of the two polymers at a weight fraction composition of 0.5 were pressed into plaques and heated for a range of times at temperatures of 543, 553, and 563 K. The small-angle-scattering cross sections were collected for the plaques on quenching to ambient temperature. These data were analyzed using a procedure based on second-order kinetics developed earlier and which has been applied to a range of polyesters. From the rate constants obtained the activation energy and preexponential factor for the transesterification reaction have been obtained. Aspects of the reaction that have been investigated were the molecular weight of the hydrogenous polymer and the ratio of hydroxyl to carboxyl end groups in a hydrogenous polymer, the molecular weight of which was close to that of the deuterio polymer. Although there is an increase in the rate constant as the average molecular weight decreases, there is no effect of molecular weight on the activation energy that has an average value of 168 kJ mol(-1). The increase in rate constant with a decrease in molecular weight indicates the importance of the role of end groups in the transesterification reaction. The relative contribution of hydroxyl or carboxyl groups to the reaction mechanism could not be discerned from these data. It is speculated that this is due to the dominant role of the hydroxyl end group in the reaction since the concentration of this varied but little in the mixtures.