a process is disclosed for the production of alkylaromatic compounds employing olefinic liquid from thermally or catalytically cracked plastics as alkylating agent . the process comprises contacting a feedstream comprising alkylatable aromatics and the olefinic liquid with acidic alkylation catalyst under alkylation conditions in an alkylation zone ; and recovering an effluent stream comprising alkylaromatic compounds . the alkylation can be performed with the product of plastics pyrolysis or with non - degraded plastic feedstock in - situ with thermal / catalytic degradation of the plastic .