An excellent method for the introduction of fluorine into the aromatic nucleus, described by Balz and Schiemann, has been extensively extended. The method involves two steps, first, the preparation and solution of a dry diazonium fluoborate, and second, the controlled decomposition of this salt by heat to yield an aromatic fluoride, nitrogen, and boron trifluoride. This phenomenon which makes possible, the Schiemann reaction is the remarkable stability of the dry diazonium borofluoride. The overall yields are satisfactory and no special apparatus is necessary. The reaction may be carried out on a wide variety of amines. The effect of the Schiemann reaction on other groups present in the ring is discussed in detail.