AbstractTransiodinationoccurs when reacting iodobenzenes with other halogeno‐or with alkylbenzene derivatives in the presence of aluminium bromide at room temperature.Isomer equilibriaamong iodobenzenes resemble those for the corresponding bromides1.Di‐ and tri‐substitutedbromo‐ and iodo‐benzenes show equilibrium patterns agreeing with those computed on the basis of addivitity, using patterns for monosubstituted halogenobenzenes. Results are discussed in terms of partial equilibrium factors1, believed to be mainly governed by electrostatic and steric interactions.Results ofdirect brominations(AlBr3catalyst, no solvent) indicate that for halogeno‐ and alkylbenzenes the main difference between the isomer ratios observed and those at equilibrium consists in the large proportions ofmeta(or 1, 3, 5) isomer found in the