A process is described for producing an alkylaromatic compound in a multistage reaction system comprising at least first and second series-connected alkylation reaction zones each containing an alkylation catalyst. A first feed comprising an alkylatable aromatic compound and a second feed comprising an alkene and one or more alkanes are introduced into said first alkylation reaction zone, having operating conditions, e.g., temperature and pressure, which are controlled effective to cause the alkylatable aromatic compound to be partly in the vapor phase and partly in the liquid phase with the ratio of liquid volume to vapor volume of the feed in each zone to be from about 0.5 to about 10. The aromatic compound and the alkene are reacted in the presence of the alkylation catalyst to form an effluent comprising the alkylaromatic compound, unreacted alkylatable aromatic compound, any unreacted alkene and the alkane, which is withdrawn and then supplied to the second alkylation reaction zone without removal of the alkane.