Abstract A hydrolytic degradation study of two bio-based polyesters, poly(trimethylene malonate) (PTM) and poly(trimethylene itaconate) (PTI) with bimodal molecular weight distribution, was performed in aqueous solutions adjusted to pH values from ∼5.5 to 11. Final weight loss varied from 20 to 37 wt% for PTM and from 7 to 21 wt% for PTI as a function of degradation time and initial solution pH. Fourier transform infrared (FTIR) spectroscopy was used to monitor ester bond concentration of these bioplastics, and the molecular weights and polydispersity index were obtained by gel-permeation chromatography (GPC). Solutions with lower initial pH values resulted in lower molecular weights for both PTM and PTI after one week of degradation. Degradation especially affected the amorphous region, leading to an increase in crystallinity of PTI samples.