A process and system for the vapor phase alkylation of an aromatic substrate in a multi-stage alkylation reaction zone having a plurality of series-connected catalyst beds spaced from one another to provide mixing zones between adjacent catalyst beds. A feedstock containing an aromatic substrate and a C.sub.2 C.sub.4 alkylating agent is supplied to an inlet side of the reaction zone. The reaction zone is operated at conditions in which the aromatic substrate is in the gas phase and causing vapor phase alkylation of the aromatic substrate as the aromatic substrate and the alkylating agent flow through the reaction zone and pass from one catalyst bed to the next. A quench fluid comprising one or both of the aromatic substrate and the alkylating agent is supplied into the interior of the mixing zone through a plurality of flow paths in which one portion of the flow paths is directed upwardly within the mixing zone and another portion downwardly within the mixing zone. Alkylated product is then recovered from the downstream side of the reaction zone. The quench fluid is supplied to the mixing zone through a plurality of dispersion channels which are spaced laterally from one another and extend transversely across the mixing chamber. At least some of the dispersion channels dispense the quench fluid alternately, upwardly and downwardly within the mixing zone.