A multistage synthesis of the catalyst for the gas-phase alkylation of benzene with ethylene is implemented. A highly active catalyst is synthesized from zeolite ZSM-5 using thermal-vapor treatment. The obtained catalyst possesses an optimal acidity, which makes it possible to achieve a higher yield of ethylbenzene compared to the commercial catalyst based on ZSM-5 (88 and 62.7 wt %, respectively), an approximately equal ethylbenzene selectivity, and an almost three times lower concentration of the undesirable xylene impurity in an alkylate (0.004 and 0.011 wt %, respectively) in the gas-phase benzene alkylation with ethylene.