The morphology of poly(ethylene terephthalate)/poly(butylene terephthalate) (PET/PBT) blends before to and after heat treatment have been studied using differential scanning calorimetry (DSC), wide and small angle x-ray scattering (WAXS and SAXS), nuclear magnetic resonance spectroscopy (NMR) and small angle neutron scattering (SANS). Blends with PET/PBT compositions of 100/0, 97/3, 90/10, 60/40, 50/50, 40/60, 25/75 and 0/100%w/w were prepared by precipitation from solutions of the two polymers at the required concentrations. Blends were heat treated to induce ester interchange reactions for a) 6 hours at 476K and b) 1/2 hour at 573K.NMR data showed that the samples iieat treated for 6 hours at 476K were block copolymers and the samples heat treated for 1/2 hour at 573K were random copolymers. DSC, WAXS and SAXS experiments established the morphology of the blends, block and random copolymers. SANS experiments were carried out to study the kinetics of transesterification of PET/PBT copolyesters. Deuterated PET has been synthesised. Data was compared for different molecular weights of deuterium-PET/hydrogenous-PBT blends prior to and after heat treatment to investigate changes in molecular weight of the deuterated chain length as a result of transesterification reactions. From these data it was possible to establish the activation energy of PBT and the results indicate that transesterification reactions take place randomly along the polymer backbone, i.e. by ester-ester interchange.