A process for manufacturing cumene by reacting propylene with benzene, including treating a vent stream from the reaction zone, to recover propylene for return to the reactor. The process involves using a gas separation membrane to separate propylene from propane in the reactor vent stream. The membrane separation step results in a residue stream typically containing as much as 30%, 40% or more propane, which is vented from the process, and a permeate stream containing 95% or less propylene, which is recirculated to the reactor.