A process for producing a halogen-containing ethylbenzene derivative represented by the general formula:
where X is a halogen atom, and Y is a nitrile group, a carboxyl group, a lower alkoxycarbonyl group or an amidocarbonyl group, which comprises reacting a vinyl compound represented by the general formula:
where Y is as defined above, with a benzenediazonium salt and halogen ions in a solvent mixture comprising a lower alcohol and/or an ether, and water, under an acidic condition with a mineral acid, in the presence of a monovalent copper compound as a catalyst.