U.v. irradiation of a mixture of sulphur chloride pentafluoride and trifluoroethylene yields a mixture (ca. 7 : 3) of 2-chloro-1,2,2-trifluoroethylsulphur pentafluoride (I) and 2-chloro-1,1,2-trifluoroethylsulphur pentafluoride (II). Both (I) and (II) give, inter alia, trifluoroethylene and chlorotrifluoroethylene when pyrolysed. Isomer (I) yields perfluorovinylsulphur pentafluoride when treated with potassium hydroxide, but isomer (II) appears to be completely destroyed by this reagent.