The base catalyst HY-BS, which is binder-free zeolite Y in the acid H+ form, has been synthesized and modified with hydrochloric and citric acid solutions. It has been shown that all the obtained catalysts exhibit high activity and selectivity in the reaction of liquid-phase alkylation of benzene with ethylene. It has been first found that the ethylbenzene concentration in the alkylate and the selectivity for ethylbenzene in the liquid-phase benzene alkylation reaction on the catalyst modified with 0.3 N hydrochloric acid are higher when either the hydrogenated or the nonhydrogenated ethane-ethylene fraction of pyrolysis is used as an alkylating agent instead of polymerization-grade ethylene.