Fluorination of uracil with acetylhypofluorite and fluorine in acetic acid is described. The influence of the acetate ion on the products of the fluorination reaction was examined. Two stereoisomers (cis (3) and trans (4)) were found in the reaction mixture following fluorination in the absence of the acetate ion, but only one isomer (4) and 5-fluorouracil were found when the acetate ion was present during the fluorination reaction. 2H nuclear magnetic resonance revealed that acetate from the solution containing acetate ion rather than from the residue from acetylhypofluorite binds to position 6 of uracil to form intermediates. The synthesis yields 5-fluorouracil isolated as pure compound in a chemical yield, relative to the fluorine introduced, of about 45%. The influence of inorganic cations on the yield of acetylhypofluorite was also evaluated.