PURPOSE: To remove the acid fluoride group from an acid fluoride group-containing perfluoro compound without using a toxic gas, in a relatively short time, and to obtain a dimer useful as a base oil of a non-reactive oil for use in high vacuum, by treating the perfluoro compound with plasma.

CONSTITUTION: An acid fluoride group-containing perfluoro compound of formula I(m is positive integer; X is F or CF3) or formula II (n is positive integer) is subjected to the plasma treatment. The acid fluoride group is eliminated and a dimer is obtained by this treatment. The plasma treatment is carried out by cooling the reaction system, evacuating to 1W3 Torr, and contacting the substance to be treated with plasma generated by the radiofrequency discharge of 13.56 MHz.

EFFECT: Polyperfluoroether having a polymerization degree of about 100 can be obtained by the above dimerization process.