A process is described for producing an alkylaromatic compound, in which a first feed comprising an alkylatable aromatic compound and a second feed comprising an alkene are introduced into a first alkylation reaction zone comprising a first alkylation catalyst. The first alkylation reaction zone is operated under conditions effective to cause alkylation of the alkylatable aromatic compound by the alkene to produce said alkylaromatic compound, the conditions being such that the alkylatable aromatic compound is at least predominantly in the vapor phase. A first effluent comprising the alkylaromatic compound and unreacted alkylatable aromatic compound is withdrawn from the first alkylation reaction zone and at least part of the unreacted alkylatable aromatic compound is treated to remove catalyst poisons therefrom and produce a treated unreacted alkylatable aromatic stream. At least part of the unreacted alkylatable aromatic compound and a third feed comprising said alkene is introduced into a second alkylation reaction zone comprising a second alkylation catalyst. The second alkylation reaction zone is operated under conditions effective to cause alkylation of the unreacted alkylatable aromatic compound by the alkene to produce said alkylaromatic compound, the conditions being such that the alkylatable aromatic compound is at least predominantly in the liquid phase. A second effluent comprising said alkylaromatic compound is withdrawn from the second alkylation reaction zone.