The present invention is related to a new method for the synthesis of alpha-aminoalkylenephosphonic acid or its phosphonate esters comprising the steps of forming a reaction mixture by mixing a P—O—P anhydride moiety comprising compound, having one P-atom at the oxidation state (+III) and the other P-atom at the oxidation state (+III) or (+V), an aminoalkanecarboxylic acid and an acid catalyst, wherein said reaction mixture comprises an equivalent ratio of alpha-aminoalkylene carboxylic acid to P—O—P anhydride moieties of at least 0.2, and recovering the resulting alpha-aminoalkylene phosphonic acid compound or an ester thereof from the reaction mixture.