Environmental challenges resulting from the increasing amount of plastics produced each year, emphasise the importance of alternative, sustainable and biodegrading materials. Aliphatic and aromatic biopolyesters are an interesting category of bioplastics, which can either be biodegradable or bio-based. The ester bonds easily degrade when exposed to the appropriate environment, which is beneficial at end of life, but is undesirable during melt processing. Exposure of ester bonds to thermomechanical loading results in a decrease in molecular weight and negatively affects the material properties. The aim of this research is to understand and characterize the degradation behaviour of a biopolyester during thermomechanical shear loading in the melt phase. Poly(lactic acid) or PLA is used as a case study and the influence of fundamental parameters (shear stress history, melt temperature history, moisture content and residence time) on the degradation of PLA is investigated. In this 24-factorial design, the molecular weight is used as an indicator for the degradation and a parallel plate rheometer is used for the measurements of three PLA-grades with a different percentage D-isomer. The results of this factorial design provide insight in the effect of fundamental parameters and allow to make the transition towards processing parameters on a single-screw extruder. The processing parameters (screw rotation speed, die temperature, screw compression ratio etc.) during extrusion are based on the same fundamental parameters and will be investigated in a later phase of this research. The knowledge created about degradation of biopolyesters is necessary to minimize degradation during melt processing and maintain material quality. This makes biopolyesters more attractive to (re)use and supports the transition towards bio-based feedstock in a circular economy.