Avantium has developed a promising technology for the production of bio-based polyethylene furanoate (PEF), which can serve as a sustainable alternative for polyethylene terephthalate (PET). PEF can be mechanically recycled with the same technology and equipment as PET. Once PEF will be recycled in a separate PEF-to-PEF recycling stream, contamination with PETis inevitable due to sorting mistakes and the use of PET recycling facilities. This study therefore investigates the influence of PET contamination during PEF-to-PEF mechanical recycling. PEF batches with 0%, 2%, 5%, and 10% PET contamination were subjected to two consecutive recycling loops. Throughout the recycling process, several characterization methods were used to determine molecular properties (molecular weight, end-groups, de gree of transesterification), material properties (mechanical properties, thermal properties, optical properties) and processing behavior (rheology, hot drawing). This resulted in an overview of the effect of mechanical recycling on PEF in general and an overview of the ef fect of different levels of PET contamination after the first and second recycling loop. Since the limitations on the material properties of PEF depend on the application and the customer, no firm conclusions can be drawn on acceptable levels of PET contamination. This study does however give an overview of the effects of different levels of PET contamination and the dependence of these effects on the degree to which PET and PEF are transesterified.