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.