A beneficial method for the manufacture of amino polyalkylene phosphonic acids, under substantial absence of hydrohalogenic acid, is disclosed. The method, in essence, is based on reacting narrowly defined ratios of phosphorous acid, an amine, a formaldehyde in presence of specific ranges of an acid catalyst having a pKa equal or inferior to 3.1. The inventive method is capable of yielding economically and quality operational/capacity advantages, in particular significantly reduced one-step cycle duration under exclusion, of corrosion disadvantages and also is environmentally friendly without requiring, in that respect, anything more than nominal capital expenditures.