The alkylation of benzene with ethylene over beta-zeolite catalyst was studied using in situ IR method at 393, 433 and 473 K. Benzene (or ethylbenzene) and ethylene were injected into in situ IR cell either sequentially or in reverse order. The evolution of IR spectra was observed and recorded. The alkylation mechanism inferred is as follows: adsorbed ethylene on weaker acidic site may react with adsorbed benzene or ethylbenzene on strong acidic sites. These substances are adsorbed competitively at the same kind of sites. Ethylene adsorbed becomes carbenium, a substance which has strong interaction with acidic sites, on the other hand, benzene and ethylbenzene adsorbed molecularly have weak interaction, in the form of hydrogen bonding, with acidic OH. If an activated ethylene molecule has too few adsorbed benzene molecules around to react to alkylation, this ethylene molecule can react with other ethylene through alkenyl carbenium ion to form an oligomer, a substance considered to be a precursor of coke. (C) 2002 Elsevier Science B.V. All rights reserved.