A process is described for producing an alkylaromatic compound by reacting an alkylatable aromatic compound with a feed comprising an alkene and an alkane in a multistage reaction system comprising at least first and second series-connected alkylation reaction zones each containing an alkylation catalyst. At least the first alkylation reaction zone is operated under conditions of temperature and pressure effective to cause alkylation of the aromatic compound with the alkene in the presence of the alkylation catalyst, the temperature and pressure being such that the aromatic compound is partly in the vapor phase and partly in the liquid phase. An effluent comprising the alkylaromatic compound, unreacted alkylatable aromatic compound, any unreacted alkene and the alkane is withdrawn from the first alkylation reaction zone and at least part of the alkane is removed from the effluent to produce an alkane-depleted effluent. The alkane-depleted effluent is then supplied to the second alkylation reaction zone.