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• A simple, sensitive & accurate analytical method for PET microplastics in sediments. • Polyester microplastics are found and quantified in marine and freshwater sediments. • The analytical protocol can be extended to complex environmental matrices. Microplastics are ubiquitous pollutants in marine and freshwater bodies. Poly(ethylene terephthalate) microfibers (PMFs) are among the main primary microplastics (as-produced polymer microparticles). Released in large amounts in laundry wastewaters, PMFs end up in freshwater and marine sediments due to their high density. PMFs are potentially hazardous pollutants for ecosystems and human health, being a deceiving food source for animal organisms at the base of the food chain (e.g. sediment and water filtrators, including edible shellfish and small crustaceans). This study describes a simple, sensitive and versatile procedure for quantifying the total mass of PET micro- and nano-particles in sediments. The procedure involves aqueous alkaline PET depolymerization with phase transfer catalysis, oxidation and fractionations to remove interfering species and pre-concentrate the terephthalic acid (TPA) monomer, and TPA quantification by reversed-phase HPLC. Recovery of TPA from a model sediment spiked with 800 ppm PET micropowder was 98.2 %, with limits of detection/quantification LOD = 17.2 μg/kg and LOQ = 57.0 μg/kg. Analyses of sandy sediments from a marine beach in Tuscany, Italy, showed contamination in the 370–460 μg/kg range, suggesting that a not negligible fraction of PET microfibers released in surface waters ends up in shore sediments. [ABSTRACT FROM AUTHOR]