An anhydrous mixture containing a liquid phase and a solids phase formed from components comprising finely-divided sodium fluoride, hexabromobenzene, and a phosphonium catalyst, and optionally a liquid aprotic solvent/diluent, is heated at a temperature and for a time at which sodium bromide and at least one hexahalobenzene containing at least two fluorine atoms are produced. Preferably, the sodium fluoride is activated by one or more of procedures specified herein.