During processing, polymers are often subjected to high temperatures and high shear rates during the (long) residence times inside the machine. This can cause the material to degrade, leading to structural changes in the polymer chain and thus an overall change of material properties. The main sorts of degradation that occur for (co)polyesters include thermal, oxidative, and hydrolytic degradation, which can lead to discoloration due to functional groups formation, crosslinking, or chain scission. Chain scission, being the main degradation mechanism for PET, refers to the rupture of polymer chains and can be induced by the presence of oxygen, water, or high temperatures. It results in smaller polymer chains, leading to a decrease of molecular weight of the polymer and consequently deterioration of material properties. This contribution examines the influence of three different viscosity grades on the degradation during processing of PETG. By analyzing the inherent viscosity and melt flow index of the material before and after processing, changes in the flow behaviour can be determined.