AbstractThe bromination of naphthalene vapour in the presence of ferric compounds on pumice as a carrier has been studied between 250° and 650°. The ratio of α‐: β‐monobromonaphthalene has been determined in the reaction product. More β‐isomer is produced under the influence of the catalyst than in the non‐catalysed bromination. The same holds also for the bromination of liquid naphthalene under the influence of ferric chloride, ferric bromide or ferrous bromide between 85° and 215°.The effect of the catalyst on the composition of the reaction product is a result of the reversibility of the reaction between bromine and naphthalene under these conditions. Although the equilibrium C10H8 + Br2 ⇌ C10H7Br + HBr, lies well over on the bromonaphthalene side, nevertheless an equilibrium between α‐ and β‐bromonaphthalene can be set up indirectly. The liquid equilibrium α‐ ⇌ β‐bromonaphthalene has been studied and the displacement of the equilibrium ratio with the temperature determined.