Chloroarylation of unsaturated compounds with arenediazonium chlorides in the presence of CuCl2 as catalyst involves intermediate formation of arenediazonium tetrachlorocuprates(II) [ArN2+](2) CuCl42-. A procedure for preparative isolation of these intermediates was developed, and they were shown to be efficient arylating agents. Reactions of [ArN2+](2) CuCl42- with unsaturated compounds gave the corresponding Meerwein products; a mechanism was proposed for these reactions. In polar solvents arenediazonium tetrachlorocuprates(II) are converted into chloroarenes, presumably through a cyclic transition state.