<p>Quantification of the polyester poly(ethylene terephthalate) (PET) in environmental samples is a particular challenge. Due to strong matrix effects by inorganic compounds, thermoanalytical methods are not recommendable for a precise quantification of PET in complex environmental matrices. It was shown that depolymerization followed by determination of chemolysis products is a good alternative. In this study, we developed a quantification method for PET based on methanolysis to terephthalic acid dimethyl ester using sodium methoxide as a catalyst, and subsequent determination via GC–MS. With poly(ethylene terephthalate-d₄), we introduce a new internal standard covering the whole analytical process. Satisfactory detection and quantifications limits (1&#xa0;and 4&#xa0;µg&#xa0;g⁻¹) as well as recoveries of&#xa0;87–117% were achieved. Tests with various PET-free natural compounds exhibited no interfering matrix effects. The newly developed method was applied for MP quantification in a variety of environmental samples such as sediments, sewage sludge, indoor dust, and water. In all these matrices, PET was present. Highest concentrations were detected in indoor dust with up to 57&#xa0;mg/g. In bottled water, PET concentrations were detected as high as 463&#xa0;ng/L. The described depolymerization method offers a straightforward approach for a reliable quantification of PET in complex environmental matrices suitable for routine analysis.</p> Graphical Abstract <p></p>