Process for the oligomerization of hexafluoropropylene oxide (HFPO) in which the sum of the dimers, trimers, tetramers, pentamers and hexamers represents less than 20% of the weight of the polymerized HFPO, the oligomers other than the dimers, trimers, tetramers, pentamers and hexamers (the remaining oligomers) having an average molecular weight ranging between 1100 and 4000 g/mol, characterized in that the process comprises the steps of contacting hexafluoropropylene oxide (HFPO) at a temperature between −30° C. to +50° C. with a catalyst system selected from potassium fluoride and di-, tri- or tetraethyleneglycol-dimethylether in the presence of a solvent which is selected from perfluorinated aliphatic or aromatic hydrocarbon with more than 3 and less than 12 C-atoms and optionally a polar group but not comprising chlorine or bromine partially fluorinated aliphatic or aromatic hydrocarbon with more than 3 and less than 12 C-atoms and optionally a polar group but not comprising chlorine or bromine at a pressure of less than 3 bars by controlled monomer addition continuous feeding or by adding the monomer in one or two portions (shots).