Abstract: A process for the manufacture of N-phosphonomethyliminodiacetic acid which comprises: 1) reacting iminodiacetic acid with phosphorous acid and a source of formaldehyde in aqueous solution in the presence of concentrated sulphuric acid; 2) filtering and recovering the N-phosphonomethyliminodiacetic acid product precipitated in stage (1); 3) recovering the filtrates from stage (2) and optionally removing a proportion of the water therefrom; 4) transferring the filtrates from stage (3) to a further reaction stage in which further iminodiacetic acid is reacted with phosphorous acid and a source of formaldehyde in the presence of sulphuric acid; and thereafter 5) repeating stages (1), (2), (3) and (4) in a plurality of re-cycles.