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Mg2+ (1), Ca2+ (2), Sr2+ (3), or Ba2+ (4) salts react with amino-tris(methylenephosphonic acid) (H6AMP) in a 1:1 ratio at low pH by replacing two of the six phosphonic acid protons, while the amino N maintains its proton, thus forming a zwitterion (H4AMP2-). All four structures are polymeric and substantially different from one another. In all structures, each phosphonate group is singly deprotonated, whereas the N atom is protonated. 1 consists of {Mg[HN(CH2PO3H)3(H2O)3]} n units. Mg2+ ions form zigzag chains bridged by two of the three phosphonate groups. The third phosphonate group is noncoordinated and involved in hydrogen bonding. 3 is a three-dimensional (3D) polymer with the composition [Sr(H4AMP)] n . The Sr-atoms are seven-coordinate, with five monohapto and one chelating AMP ligands. In the 3D polymer 4, {Ba(H4AMP)(H2O)} n , the two crystallographically independent Ba2+ centers are 9- and 10-coordinated by phosphonate and H2O oxygen atoms. All bridging oxygen atoms belong to phosphonate moieties that act as chelates for one Ba2+ and form 4-membered rings. The crystal structure of {Ba(H4AMP)(H2O)} n is characterized by the complete absence of hydrogen bonds.