To shorten neutralization time and treat an increased amount of a reaction mixture in a unit time using vessels of a small capacity by washing the reaction mixture of a specific olefin with an (alkyl)benzene in the presence of aluminum chloride with water, then neutralizing the oil layer separated after washing with water.

In this neutralization process, (A) a 2-4C olefin is allowed to react with (B) benzene and/or alkylbenzene in the presence of an aluminum chloride catalyst, and the resultant reaction mixture is washed with water, the oil layer is separated, and an aqueous alkali, preferably an aqueous sodium hydroxide solution of 3-10wt.% concentration, is added to effect the neutralization. The neutralization is required to maintain the oil layer at 80-120°C, preferably 90-110°C. The oil layer after the neutralization is fractionated by distillation and the objective alkylbenzene is collected. At the same time, unreacting benzene, alkylbenzenes other than objective alkylbenzene are recycled to the first reaction process and subjected to alkylation or transalkylation.