Perfluoropolyethers having an acid end group of the type of acyl fluoride (or chloride) or a carboxylic group or derivatives thereof, the other end group being a perfluoroalkyl containing one or two halogen atoms different from fluorine, are prepared by photooxidation of C.sub.3 F.sub.6 and/or C.sub.2 F.sub.4, in the presence of a fully halogenated ethylene containing 1 to 4, preferably 1 or 2 atoms of halogen different from fluorine, and by the subsequent thermal of photochemical decomposition treatment of the peroxide groups which are present in the photooxidation product.
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