For the preparation of fluorinated alkyl sulphonyl halides having the general formula: where R I f is fluorinated alkyl group and R II f is a fluorinated alkyl group or fluorine, X is a halogen atom, and Y is a proton, a halogen atom, or a fluorinated alkyl groups,    by reaction of a corresponding fluorinated unsaturated hydrocarbon with sulphuryl halide, wherein the reaction is carried out in the presence of at least catalytic amounts of a fluoride in a solvent comprising compounds having one or more SO and/or SO 2 groups incorporated in the compound structure.