Aliphatic polyesters are the most attractive candidates for biodegradable polymers. To improve their physical properties while maintaining biodegradability, incorporating aromatic terephthalate unit into the main chain of aliphatic polyesters, e.g. as in PBST, poly(butylene succinate-co-terephthalate), seems to be a promising strategy. Studies have shown that the monomer sequence distribution in such copolyesters is critical for their performance. However, what has not been addressed is the distribution of the comonomer units between crystalline and amorphous regions. This proposal aims to address this issue by performing wide-angle neutron fibre diffraction (WANFD) experiments on labeled (deuteration of the succinic unit) and unlabeled PBST copolymers.