The process is provided for forming the reaction product comprising the homologue mixture of I(CF2)nI, wherein n is 3 to 7, which may contain at least one of the contaminants ICF2I and I(CF2)2I, by the steps comprising (a) reacting iodine with hexafluoropropylene oxide at a temperature of 150° C. to 210° C. in a reactor, the amount of said hexafluoro-propylene oxide being a portion of the total amount of hexafluoropropylene oxide to be reacted with said iodine, thereby forming a reaction product containing gaseous perfluoroacetyl fluoride as a reaction by-product, (b) cooling said reaction product to become liquid except for said gaseous perfluoroacetyl fluoride, (c) venting said perfluoroacetyl fluoride from said reactor, and (d) repeating said steps (a), (b), and (c) until said total amount of said HFPO is reacted with said iodine.