The present invention relates to markers for liquids that can be used to authenticate the origin and genuineness of a liquid, preferably a bulk liquid such as fuel. For this purpose, the present invention teaches the use of a polymer capable of forming a semiconducting polymer particle (pdot) in the liquid in small amounts. The invention encompasses a liquid comprising a) a polymer that is capable of forming a semiconducting polymer particle (pdot), the concentration of the polymer being 10 ppm by weight or less, and b) an organic substance in an amount of 90% by weight or more. The invention furthermore envisages the use of such a polymer as an authenticating marker in a liquid, preferably a fuel, and a method for authenticating the genuineness and/or origin of a liquid comprising a polymer capable of forming a semiconducting polymer particle (pdot), comprising the steps i. concentrating, isolating and/or extracting the polymer capable of forming a semiconducting polymer particle (pdot); ii. aggregating the polymer or the polymer obtained in Step i. to form semiconducting polymer dots (pdots); iii. irradiating the formed pdots with electromagnetic radiation capable of exciting the pdots to emit electromagnetic radiation by fluorescence and/or phosphorescence, and iv. observing the electromagnetic radiation emitted in response to the exciting irradiation of step iii.