The adducts [RhX(1,5-C8H12)]2,2HgX2(X = Cl or Br) have been prepared by the action of mercuric halide on [RhX(1,5-C8H12)]2. Mercuric chloride and the mononuclear complexes RhCl(1,5-C8H12)L (L = Ph3P or piperidine), also give the adduct [RhCl(1,5-C8H12)]2,2HgCl2, the other product of reaction being a mercuric chloride complex of the ligand L. However, when L is Ph3As or p-toluidine the adducts RhCl(1,5-C8H12)L,2HgCl2 are formed. All the adducts dissociate in solution and the mercury halide–rhodium complex interaction is weak. I.r. (400–200 cm.–1) of the products are reported.