Conversion of benzene to heavier aromatics by contact with alkyl polynucleararomatics, preferably FCC heavy cycle oil, in the presence of an alkylation/transalkylation catalyst is disclosed. Efficient conversion of relatively dilute benzene in reformate is possible. Use of alkyl polynucleararomatics as a source of alkyl groups, with reduced use of monocyclic alkyl aromatics, permits robust reaction conditions to be used without a net formation of benzene by dealkylation. The process preferably uses a solid zeolite based acidic catalyst disposed in a fixed, moving or fluid bed reactor. Preferred catalysts comprise MCM-22 or ZSM-5.