Polyester (PET) textiles release millions of microfibers (up to 1-2 grams for fleece!) upon each washing cycle . These non-degradable filaments ultimately find their way to canals, rivers, the sea and sealife, including species for human consumption. The full extent of the harm to organisms or public health caused by microfilament exposure is still unknown. Although millions of tons of PET fibers are present, they are hardly ever detected (let alone quantified) in the vast publications on microplastics in marine environments. The reason for this “missing PET” lies in the combination of sampling, extraction and analytical methods used, which currently fail for PET. Using Dutch samples, we will develop the following innovations: • develop sampling and drying methods that quantitatively retain PET single filaments (Ø~10μm) • eliminate extraction methods (they are not suitable for microscopic PET fibres/filaments) • Introduce novel in full-matrix quantitative PET microplastic depolymerisation by transesterification with methanol/ethanol to dimethyl/ diethylterephthalate, followed by advanced GS-MS and/or LC-MS (down to 1 ppb) quantification (adapted phthalate methodology). • If possible we will try to integrate depolymerisation and analysis by further developing chemothermolysis by in-matrix in-line (in the liner) PET depolymerisation by transesterification. • The result is a high quality, quantitative, validated, fast/cheap, automatable method for PET analysis, eliminating false positives and negatives. The objective is to make this a global standard PET analysis method. The new method will also be applied to marine organisms (e.g. mussels) and food products. Ultimately, the understanding of the occurrence, fate and effect of PET microfilaments will aid in the design of textiles, washing machines and future (bio-based) polymer materials that are profitable and harmless at the same time.