4,5,6,7-Tetrafluorobenzo[ b]thiophen-2-carboxylic acid ( IA) was prepared by a modified literature procedure in two steps from pentafluorobenzaldehyde and rhodanine. Its methyl ester ( IB) was synthesised via the acid chloride ( IC) by reaction with methanol. Oxidation of IB with either CF 3CO 2H/H 2O 2 or meta-chloroperbenzoic acid gave methyl 2,3-dihydro-2,3-epoxy-4,5,6,7-tetrafluorobenzo[ b]thiophen-1,1-dioxide-2-carboxylate ( II). Methyl 2,3-dichloro- 2,3-dihydro-4,5,6,7-tetrafluorobenzo[ b]thiophen-2-carboxylate ( IIIA) was prepared by reaction of IB either with chlorine, or with sulphuryl chloride and azobisisobutyronitrile. Chloromethyl 4,5,6,7-tetrafluorobenzo[ b]thiophen-2-carboxylate ( V) was also produced with the latter reagents. Compound IIIA underwent thermal dehydrochlorination to give methyl 3-chloro-4,5,6,7-tetrafluorobenzo[ b]thiophen- 2-carboxylate ( IVA), behaviour also observed during analyses involving gas chromatography. Treatment of IB with iodine pentafluoride in CF 2ClCFCl 2 and CF 3Cl gave, as shown by GLC, one major and several minor compounds. Analyses by coupled GLC-MS and GLC-IR methods indicated that the major product was IVA. Two of the minor products appeared to be methyl 3-iodo- ( IVB) and methyl 3-fluoro-4,5,6,7-tetrafluoro- benzo[ b]thiophen-2-carboxylate ( IVC). It is not known if these were produced during analysis or in the IF 5 reaction.