A polyarylidene compound, for example biphenyl, is reacted with iodine in the presence of an oxidant (e.g. ammonium persulfate), unreactive with the aromatic muc- leus, in a reaction medium containing a solvent (e.g. an organic acid) for the polyarylidene compound, a catalyst (e.g. a mineral acid) and water. The reaction medium is recovered for reuse in repeat diiodination, by the removal of spent oxidant which, in the preferred embodiment, is based upon the temperature dependent differences in solubility between the diiodo derivative and the spent oxidant.