Aminoalkylphosphonic acid esters of general formula I, wherein R5 is a lower alkyl radical, R6 is a lower alkoxy radical and R1 is preferably a hydrogen atom, can be metallised with strong bases, for example n-butyl lithium, on the ??-carbon atoms and alkylated with alkylation agents, for example methyl iodide (Synthesis, 336/1977): ##STR11##