Acetyl hypofluorite (1) is a new compound that serves as a novel electrophilic fluorinating agent. It is special in the sense that, while it is very reactive, it is still a milder reagent than other fluoroxy compounds such as CF3OF or CF3COOF. It is synthesized directly from elemental fluorine and is used without any isolation or purification. The hypofluorite 1 reacts efficiently and selectively with activated aromatic rings, particularly phenol and aniline derivatives after suitable protection of the hydroxyl and the amino groups. The net result of the reaction is partly according to classical aromatic electrophilic substitution. Unlike such a substitution, however, the electrophilic fluorine atom of 1 substitutes mainly an ortho hydrogen and only occasionally small amounts of p-fluoro derivatives are found. Evidence supports the mechanism for this aromatic fluorination as being mainly an addition-elimination one. In many cases the electrophilic aromatic fluorinations can replace the classical 60-year-old Balz-Schiemann method, which until today is probably the most used procedure. Since aromatic fluorination with 1 is a very fast reaction and since 1 is produced directly from elemental fluorine, this is probably one of the best ways for introduction of the short-living radioisotope 18F into activated aromatic rings. This will greatly encourage the synthesis of compounds suitable for use in the rapidly developing field of positron emitting transaxial tomography, which in itself depends on the efficient and easy supply of compounds possessing positron emitting isotopes.