Fluorination of dichloromaleic anhydride by sulphur tetrafluoride gives, in significant quantities, cis- and trans-2,3-dichlorohexafluorobut-2-ene as well as 3,4-dichloro-2,2,5,5-tetrafluoro-2,5-dihydrofuran (II) and 3,4-dichloro-5,5-difluoro-2,5-dihydrofuran-2-one (III) which have previously been reported. The reaction is difficult to regulate, probably because the sulphur tetrafluoride can contain large proportions of thionyl fluoride which, although less effective, will itself fluorinate dichloromaleic anhydride to give the same mixture of products.