Thallic trifluoromethanesulfonate has been prepared by reaction of trifluoromethanesulfonic acid with either thallic oxide or solutions of thallic trifluoroacetate in trifluoroacetic acid, and has been isolated as the trihydrate on crystallization from nitromethane. The polyfluoroarenes, p-CH3OC6F4H, p-CH3C6F4H, m-H2C6F4, o-H2C6F4, p-H2C6F4, C6F5H, 1, 3, 5-F3C6H3or 1, 2, 4-F3C6H3, have been thallated by thallic trifluoromethanesulfonate in trifluoroacetic acid, giving the corresponding poly-fluorophenylthallium(iii) bis(trifluoromethanesulfonates), which have been characterized spectroscop-ically and by conversion into the corresponding (polyfluoro)iodobenzenes on treatment with sodium iodide. Only slight thallation of m-BrC6F4H and m-O2NC6F4H was observed. By contrast with thallic trifluoromethanesulfonate, thallic trifluoroacetate in trifluoroacetic acid induced little thallation of the most reactive polyfluorobenzene, p-CH3OC6F4H. Thallic trifluoromethanesulfonate also thallated p-CH3OC6F4H in nitromethane, sulfolane, and with the substrate as solvent, but p-CH3C6F4H failed to react. Treatment of the polyfluorophenylthallium(iii) bis(trifluoromethanesulfonates) with boiling aqueous sodium acetate caused symmetrization into the corresponding acetatobis(polyfluorophenyl)thallium(iii) compounds. A similar reaction between 2, 4, 6-trifluoro-phenylthallium(iii) bis(trifluoromethanesulfonate) and sodium acetate at room temperature yielded 2, 4, 6-trifluorophenylthallium(iii) diacetate. 2, 3, 4, 6-Tetrafluorophenylthallium(iii) bis(trifluoro-methanesulfonate) was converted by boiling water into bis(2, 3, 4, 6-tetrafluorophenyl)thallium(iii) trifluoromethanesulfonate.