In producing synthetic hydrocarbon oils by polymerization of olefins using aluminum halide as the catalyst, organic halides are produced. These are corrosive to metal equipment and are poisonous to certain hydrogenation catalysts. This invention is concerned with reacting such organic halides with an aromatic hydrocarbon in a system also containing the polyolefins thus forming an alkylation product with both reactants prior to removal of the aluminum chloride catalyst.