The polyiodobenzenes C6I5X (X = NO2, H, Br, Cl, F, MeO, Me, CO2H or CF3) and hexaiodo-benzene have been prepared in good yield by reaction of triiodide ions with suitable permercurated arenes in dimethylformamide at room temperature. Mass and/or p.m.r. spectra showed that singly recrystallized samples of C6I5X (X = NO2, Br, Cl, F or CF3) contained minor amounts of the corresponding tetraiodobenzenes C6HI4X. In cases where further recrystallization failed to remove the partially iodinated species, purer products could be obtained from permercurated arenes prepared using increased Hg(O2CCF3)2/substrate ratios. The polychlorobenzenes C6Cl5X (X = NO2, H, Br, F, MeO, Me or CO2H) and hexachlorobenzene have been prepared by reaction of chlorine with solutions or suspensions of the appropriate permercurated benzenes in dimethylformamide at room temperature. Attempted permercuration of naphthalene followed by chlorodemercuration gave perchloronaphthalene containing a substantial impurity of heptachloronaphthalene. Permercuration of sodium p-toluenesulphinate was accompanied by complete desulphination giving permercurated toluene.