The compounds studied in this paper, viz. (1-ammonio-1-phosphonopropyl)phosphonate, C₃H₁₁NO₆P₂, (I), and 1-(acetylamino)propylidene-1,1-bisphosphonic acid dihydrate, C₅H₁₃NO₇P₂·2H₂O, (II), are members of a commonly used family of therapeutic agents. Compound (I) is an inner salt with separated negative (on the ionized PO₃ group) and positive (on the tetrahedral N atom) charges, while (II) possesses neutral phosphonyl groups and one amide N atom. Both structures have a C-C-C-N backbone, which has comparable geometric parameters in (I) and (II); the main difference was found in one of the N–C–P bond angles, which is lengthened in (II) because of an intramolecular O_PO3–H⋯O_C=O interaction. The hydrogen-bonding scheme in the crystal of (I) includes all possible donor atoms, namely all the H atoms of the ammonium group and the phosphonic acid functions. As a result of these interactions, the zwitterions are organized into a plane running along the crystallographic x axis. In (II), the intermolecular interactions include all possible donor atoms, except for the N atom; the packing differs from that of (I) in that the molecules are arranged in a chain running parallel to the x axis. In the chains, the molecules form head-to-head dimers, while the crystallization water molecules contribute to the intra- and interchain cohesion.