A process for the continuous preparation of ethylbenzene in a heterogeneous-phase reaction by alkylating benzene with ethylene in the presence of a catalyst based on a complex organic aluminum chloride compound and a co-catalyst at an elevated temperature and elevated pressure, with a residence time of from 5 to 60 minutes in one or more reaction zones, the reaction mixture, after completion of the reaction, being fed into a pressure vessel and vigorously mixed therein for from 20 to 60 minutes at from 120.degree. to 160.degree. C.