A process for the production of ethylbenzene by the ethylation of benzene in the critical phase in a reaction zone containing a molecular sieve aromatic alkylation catalyst comprising cerium-promoted zeolite beta. A polyethylbenzene is supplied into the reaction zone and into contact with the cerium-promoted zeolite beta having a silica/alumina mole ratio within the range of 20-500. The reaction zone is operated at temperature and pressure conditions in which benzene is in the supercritical phase to cause ethylation of the benzene and the transalkylation of polyethylbenzene and benzene in the presence of the zeolite beta catalyst. An alkylation product is produced containing ethylbenzene as a primary product with the attendant production of heavier alkylated byproducts of no more than 60 wt. % of the ethylbenzene. The alkylation reaction zone is operated under conditions providing a composite byproduct yield of propyl benzene and butyl benzene relative to ethylbenzene, which is no more than one half of the corresponding yield byproduct for zeolite beta promoted with lanthanum. The production of ethylbenzene in the critical phase alkylation reaction zone is attended by recycle of a polyalkylated aromatic component of the reaction product back to the reaction zone.