Polyester is a synthetic polymer that is very easy to recycle, if it is available in large quantities, sorted by type and with only minor contamination (this excludes any materials from the household waste stream). Used bottles made of PET from the food industry are ideal for recycling. Many manufacturers and retailers promote this recycled polyester (rPET) as environmentally friendly and sustainable, since using recycled polyester from melted down and spun PET bottles uses fewer resources than producing new fibers. However, it should be noted that the use of this material is «down cycling,» as the PET material is removed from the stream and is no longer primarily used to make new drinking bottles or food packaging. The use of textiles made of polyester fibers for the recovery of recycled polyester is also possible and is increasingly being realized on the market. Here, however, the necessary effort for sorting and cleaning is significantly greater and the existing market is much smaller. Due to the increasing global demand for recycled polyester from the food packaging industry (the mandatory use of recycled polyester in beverage bottles is growing due to implementation of Single Use Plastic Directives) and from apparels manufacturers, significantly higher and further increasing prices are demanded for the recycled material than for so called “virgin” PET. Against this background, it is logical to check whether the material offered or used is actually a containing the declared amount of recycled polyester.