Abstract: An essentially anhydrous agitated mixture of (i) finely-divided essentially anhydrous alkali metal fluoride (e.g., KF), (ii) haloaromatic compound having at least one halogen atom of atomic number greater than 9 on an aromatic ring (e.g., C6Cl6), and (iii) an organo guanidinium salt catalyst (e.g., [(Et2N)2CNEt2]Br), is heated at a temperature at which fluorine replaces one or more of the ring halogen atoms of the haloaromatic compound. A new synthetic route to hexahydrocarbyl guanidinium salts is also described.