tetrafluoride with 1,4-dioxan in anhydrous hydrogen fluoride in the presence of small additions of sulfur chlorides or chlorine results in the replacement of several hydrogen atoms in dioxan ring by fluorine atoms. Depending on various conditions, 2,3,3-trifluoro-1,4-dioxan, 2,3,3,5,6-pentafluoro-1,4-dioxans as well as 1,2,2,2,2-pentafluoroethyl ether can be obtained as major products in a good yield.