Isomeric Mannich bases derived from unsymmetrical ketones can be synthesised regioselectively simply by selecting the reaction conditions; reaction of dimethyl(methylene)ammonium trifluoroacetate in trifluoroacetic acid yields the more substituted aminoketone while reaction of di-isopropyl(methylene)ammonium perchlorate in acetonitrile leads to the less substituted aminoketone.