Process for preparation of impact-resistant polystyrene from diene monomers and from styrene monomers via anionic polymerization, where 1) in a stage 1) a rubber solution is prepared from the diene monomers, or from the diene monomers and from the styrene monomers, using an alkali metal organyl compound as initiator and with concomitant use of a solvent, and then 2) in a stage 2), styrene monomer is added to the rubber solution, and the resultant mixture is polymerized anionically to give the impact-resistant polystyrene, and where, after stage 1) and prior to stage 2), an organylaluminum compound and an alkali metal hydride are added to the rubber solution.