Six piperazinium salts, namely 4-(4-nitrophenyl)piperazin-1-ium 4-bromobenzoate dihydrate, C10H14N3O2+⋅C7H4BrO2-⋅2H2O, (I), 4-(4-nitrophenyl)pipera- zin-1-ium 4-iodobenzoate dihydrate, C10H14N3O2+⋅C7H4IO2-⋅2H2O, (II), 4-(4-nitrophenyl)piperazin-1-ium 4-hydroxybenzoate monohydrate, C10H14N3O2+⋅- C7H5O3-⋅H2O, (III), 4-(4-nitrophenyl)piperazin-1-ium 4-methylbenzoate monohydrate, C10H14N3O2+⋅C8H7O2-⋅H2O, (IV), 4-(4-nitrophenyl)piperazin-1-ium 4-methoxybenzoate hemihydrate, 2C10H14N3O2+⋅2C8H7O3-⋅H2O, (V), and 4-(4- nitrophenyl)piperazin-1-ium 4-ethoxybenzoate, 2C10H14N3O2+⋅2C9H9O3-, (VI), have been synthesized and their crystal structures solved by single-crystal X-ray diffraction, revealing that all of them crystallize in the triclinic space group P1 except for (V), which crystallizes in the monoclinic space group P21/c and has a disordered nitro group. Compounds (I) and (II) are isostructural. The crystal packing of (I)-(V) is constructed from organic chains formed by a combination of hydrogen bonds of type N--H⋯O and/or O--H⋯O and other weak interactions of type C--H⋯O and/or C--H⋯π, forming sheets, whereas (VI) shows a cationic and anionic-based layer structure. [ABSTRACT FROM AUTHOR]