A process for the production of styrene monomers by depolymerization of a polystyrene-containing polymer composition, comprising the steps of: a) introducing a polymer composition (A) containing: I) 60% to 99.5% by weight of polystyrene (I), based on the total weight of the polymer composition (A); and II) 0.1% to 30% by weight of polyolefin (II), based on the total weight of the polymer composition (A); and/or III) 0.1% to 4.9% by weight of acrylonitrile-based polymer (III), based on the total weight of the polymer composition (A); and/or IV) 0.1% to 4.9% by weight of polyester (IV), based on the total weight of the polymer composition (A), into the reaction zone (R) of a pyrolysis reactor (P); b) thermally cleaving the polystyrene present in the polymer composition (A) in the reaction zone (R) of the pyrolysis reactor (P) at a temperature of from 400°C to 1000°C, in order to obtain a product mixture (G) containing styrene monomers and further components; c) withdrawing the product mixture (G) obtained in step b) from the reaction zone (R) of the pyrolysis reactor (P); d) cooling the product mixture (G) withdrawn in step c), in order to obtain a condensed product mixture (K) containing styrene monomers and further components; and e) separating the styrene monomers from the further components of the condensed product mixture (K) obtained in step d), wherein the average residence time (Z) of the polymer composition (A) in the reaction zone (R) of the pyrolysis reactor (P) is from 0.01 s to 10 s. a) introducing a polymer composition (A) containing: I) 60% to 99.5% by weight of polystyrene (I), based on the total weight of the polymer composition (A); and II) 0.1% to 30% by weight of polyolefin (II), based on the total weight of the polymer composition (A); and/or III) 0.1% to 4.9% by weight of acrylonitrile-based polymer (III), based on the total weight of the polymer composition (A); and/or IV) 0.1% to 4.9% by weight of polyester (IV), based on the total weight of the polymer composition (A), into the reaction zone (R) of a pyrolysis reactor (P); I) 60% to 99.5% by weight of polystyrene (I), based on the total weight of the polymer composition (A); and II) 0.1% to 30% by weight of polyolefin (II), based on the total weight of the polymer composition (A); and/or III) 0.1% to 4.9% by weight of acrylonitrile-based polymer (III), based on the total weight of the polymer composition (A); and/or IV) 0.1% to 4.9% by weight of polyester (IV), based on the total weight of the polymer composition (A), b) thermally cleaving the polystyrene present in the polymer composition (A) in the reaction zone (R) of the pyrolysis reactor (P) at a temperature of from 400°C to 1000°C, in order to obtain a product mixture (G) containing styrene monomers and further components; c) withdrawing the product mixture (G) obtained in step b) from the reaction zone (R) of the pyrolysis reactor (P); d) cooling the product mixture (G) withdrawn in step c), in order to obtain a condensed product mixture (K) containing styrene monomers and further components; and e) separating the styrene monomers from the further components of the condensed product mixture (K) obtained in step d), <EOS>