An improved method for the manufacture of phosphonoalkyl iminodiacetic acid M2PO3—X—N—(CH2COOM)2 wherein X is a C1-6 linear or branched alkyl group; and M is selected from hydrogen, alkali, earth-alkali, ammonium and protonated amine is disclosed. The iminodiacetic acid starting material is reacted with a substantially stoichiometric amount of phosphorous acid, in the presence of a large excess of phosphoric acid to thereby yield a reaction medium insoluble reaction product (PAI-DA) which can be separated from the reaction medium. In a particularly preferred approach, the phosphorous acid is prepared in situ starting from liquid P4O6.