Abstract: A process for the preparation of flameproofed polystyrene whicha) contains halohydrocarbons as flameproofing agents and which additionally contains one or more agents from the group ofb) synergists for the flameproofing agent,c) stabilizers,d) antioxidants ande) additivesby mixing polystyrene with one or more of agents a) to e) in a plurality of stages, wherein in a first stage the flameproofing agent a) and the entire amount or a proportion of the stabilizers c) are premixed at up to 100.degree. C. in a mixing apparatus and this premix is then, in a second stage, mixed with the other agents, in a continuously operated single-screw or twin-screw extruder or a kneader of conventional construction, in such a manner that, viewed in the conveying direction of the extruder, first the premix, with or without other agents, is continuously fed in and thereafter, in the further mixing zones, a melt of a polystyrene resin, which has been substantially freed from monomers and solvents and is at from 200.degree. to 250.