The present invention relates to novel .omega.-fluorosulfato-perfluorocarboxylic acid derivatives of the formula I ##STR1## in which A denotes halogen, preferably Cl or F, and in particular F, or the group OR wherein R is alkyl, aryl or aralkyl with preferably up to 10 C atoms, in particular CH.sub.3 or C.sub.2 H.sub.5, m denotes a number from 1 to 10, preferably from 1 to 8 and in particular from 1 to 6, and n denotes a number from 0 to 10, preferably from 0 to 4 and in particular 0 or 1, which are prepared by electrolysis of .omega.-H-perfluorocarboxylic acid halides of the formula II ##STR2## in which A'=halogen and m and n have the same meaning as in formula I, in an electrolyte consisting of fluorosulfonic acid/alkali metal fluorosulfonate, the compounds of the formula III being formed: ##STR3## in which A' has the same meaning as in formula II and m and n have the same meaning as in formulae I and II, these compounds then also being esterified with an organic hydroxy compound of the formula IVROH (IV)in which R has the meaning given in the case of formula I, to give compounds I in which A=OR.The compounds of the present invention are valuable intermediate products particularly suitable for the preparation of perfluorinated vinyl compounds with an additional functional group wherein the vinyl compounds can in turn be processed to valuable oligomers and polymers which are very stable to heat and chemicals and can be used in many ways.