Abstract α-Dialkylaminomethylphosphines and the corresponding arsines have been obtained by the following methods: (1) by direct Mannich reaction, (2) by reaction of secondary phosphines and arsines with bis(dimethylamino)methane, and (3) by reaction of methyleneammonium halides with secondary phosphines and arsines, silylphosphines and silylarsines, and alkali phosphides and alkali arsides, respectively. The compounds obtained show a similar reactivity as N, N-acetals. The aminomethyl group is transferred to H-acidic agents, the reaction with halides yields methylene ammonium halides and halophosphines or haloarsines, respectively. Reactivity towards methyl iodide is influenced by substituents at the phosphorus or arsenic atoms. In the case of the phosphorus compounds acetylphosphines are obtained by reaction with acetyl chloride, while the arsenic compounds add acetyl chloride to give ammonium salts.