FIELD: chemistry.SUBSTANCE: present invention relates to a method of synthesis of aminoalkylene phosphonic acid or its phosphonate esters and can be used in chemical industry. Proposed method comprises the steps of: a) forming, in the presence of aldehyde and acid catalyst, a reaction mixture by mixing compound (a.1.) containing at least one HNR1R2 fragment, or a salt thereof with a compound (a.2.) containing one or more anhydride P-O-P fragments, wherein fragments contain one atom P in degree of oxidation (+III) and one atom P in degree of oxidation (+III) or (+V), where ratio of moles of aldehyde to N-H fragments is 1 or more, and where ratio of fragments of N-H to anhydride fragments P-O-P is 0.3 or more, and where the aldehyde has the general formula R-CO-H, where R is selected from a group consisting of hydrogen, an aliphatic moiety, an araliphatic fragment, an aromatic moiety and a heterocyclic moiety, where total number of carbon atoms and heteroatoms ranges from 1 to 11, compound (a.1.) is characterized by that (a.1.1) R1 and R2 are independently selected from hydrogen, C1-C6alkyl, C1-C6alkenyl, C1-C6alkynyl, C1-C6acyl, optionally containing ethylenically unsaturated double bond, and (meth)acryloyl C1-C6-fragment, wherein C1-C6-part of said fragments has a normal, branched or cyclized chain and optionally substituted with one or more fragments selected from C1-C4hydrocarbon, aryl and aralkyl, and optionally contains one or more heteroatoms selected from oxygen, nitrogen and sulfur, and optionally contains one or more carbonyl moieties, or (a.1.2) R1 and R2 form a circular structure, where N-H is included in said ring, and where said ring is optionally substituted with one or more fragments selected from C1-C4hydrocarbon, aryl and aralkyl, and optionally contains one or more heteroatoms selected from oxygen, nitrogen and sulfur, and optionally contains one or more carbonyl fragments, and where compounds (a.2.) containing anhydride P-O-P fragments which contai
