The objective of this study was to develop a mathematical model that describes and even predicts the hydrolytic degradation of aliphatic polyesters. From literature, it is known that the main process of degradation of aliphatic polyesters is the autocatalytic hydrolysis of ester bonds. Because of this hydrolysis, polymer chains are cleaved and the molecular weight will decrease. With time, the molecules become small enough for the system to start losing weight, since the small molecules dissolve in the aqueous medium. In addition, the crystallinity can change during degradation and influence the degradation rate of the polymer and the monomer ratio of a copolymer. In order to test the model several aliphatic polyesters were synthesized. The degradation behavior of these polymers was investigated by placing them in an aqueous environment (pH = 7.4) at 37 degrees C, At certain time intervals, samples of the polymers were taken and analyzed. A mathematical model was developed based on the autocatalytic hydrolytic degradation mechanism of aliphatic polyesters and verified with the measured results. Up to now, no models are known that include the autocatalytic hydrolysis behavior of aliphatic polyesters. The calculations of the new developed model are compared with measured results. It shows that it describes the concentration change of all polymer chains present as function of the degradation time. Hence, the change in molecular weight distribution, the decrease of the average molecular weight and the mass loss of the polymer as function of the degradation time can be predicted. This is a major advancement with respect to any earlier developed model. The only unknown input parameter for the model is the hydrolysis rate constant. The mathematical model is valid for semicrystalline as well as amorphous polymers and for copolymers.