Perfluorocarbonyl compounds of the formulae IV and V ##STR1## in which R.sub.f denotes perfluoroalkyl, R'.sub.f denotes perfluoroalkylene, R denotes F, Cl, perfluoroalkyl or --OR"' (R"'=alkyl, aryl or aralkyl) and n denotes a number from 0 to 10, are prepared, starting from compounds of the formula I ##STR2## in which R.sub.f, R'.sub.f and n have the same meaning as in the formulae IV and V and R" is F, Cl or perfluoroalkyl, by electrolysis in an electrolyte consisting of fluorosulfonic acid/alkali metal fluorosulfonate and, if necessary, esterification with R"'OH, and decomposition ##STR3## in the presence of catalytic amounts of alkali metal fluorides and/or aprotic N-bases. Most of the compounds IV and V are new; the latter are, above all, valuable heat transfer fluids, lubricating oils or intermediates for the preparation of other F-organic compounds.