The transesterification reaction between polyethylene terephthalate and polybutylene terephthalate (PBT) was investigated by small-angle neutron scattering. For this purpose deuterio polyethylene terephthalate (dPET) was synthesised and fractionated to produce three samples whose molecular weight ranged from circa 14 000 g mol(-1) to 63 400 g mol(-1). Mixtures of these two polymers containing 10% of the deuterio polymer were transesterified to varying extents by heating for known times at different temperatures. The apparent molecular weight of the deuterio polymer decreases as the extent of reaction increases and this is observed in the small-angle scattering data. From these data second order rate constants were obtained, the values are somewhat larger than those obtained for main-chain mesomorphic polyesters but of the same order of magnitude for transesterification in polyethylene terephthalate alone. No influence of the large difference in molecular weight of the dPET on the rate constants was observed. The activation energy obtained from these data was 61 +/- 7 kJ mol(-1), a value which is less than half that reported for other polyester transesterifications. The reaction appears to be dominated by the scission of the PET component of the mixture.