PURPOSE: To obtain the subject compound useful as an emulsion, etc., for polymerization of fluoroolefin by reacting a perfluoroalkyl iodide with fuming sulfuric acid.

CONSTITUTION: (B) A perfluoroalkyl iodide of the formula RfI (Rf is a 2-17C perfluoroalkyl) is reacted with (C) fuming sulfuric acid containing 50-70% sulfur trioxide in a liquid phase in the presence of one or more catalysts comprising a group of compounds of elements of P, Mo and Sn at 50-200°C reaction temperature to afford (D) a perfluoroalkylcarboxylic acid fluoride of the formula R'fCOF (R'f is a 1-16C perfluoroalkyl) and the resultant reaction mixture is separated into an organic phase containing the component D and an inorganic phase consisting essentially of sulfuric acid, and the organic phase containing the component D is rectified to provide the objective fluoride. Furthermore, one or more kinds of catalysts comprising a group of phosphorus compounds, e.g. PCl5, P2O5 and antimony pentachloride are preferably used as the component A.