The invention relates to a method for producing styrene monomers by the depolymerisation of polystyrene in the presence of foreign polymers, such as polyolefins. Said method comprises the following steps: a) introducing a polymer composition (A) containing: I) 10 to 99.5% by weight, based on the polymer composition (A), of polystyrene (I); and II) 0.1 to 89.9% by weight of polyolefin (II); and/or III) 0.1 to 4.9% by weight of acrylonitrile-based polymer (III); and/or IV) 0.1 to 4.9% by weight of polyester (IV), into the reaction zone (R) of a pyrolysis reactor (P); b) thermal cracking the polystyrene contained in the polymer composition (A) in the reaction zone (R) of the pyrolysis reactor (P) at a temperature of between 400-1000° C., c) removing the product mixture (G) obtained from the reaction zone (R), d) cooling of the product mixture (G), and e) separating the styrene monomers from the further components.