Crystal structures are reported for N-(4-meth­oxy­phen­yl)piperazine (MeOPP), (I), and for its 3,5-di­nitro­benzoate, 2,4,6-tri­nitro­phenolate (picrate) and 4-amino­benzoate salts, (II)–(IV), the last of which crystallizes as a monohydrate. In MeOPP, C₁₁H₁₆N₂O, (I), the 4-meth­oxy­phenyl group is nearly planar and it occupies an equatorial site on the piperazine ring: the mol­ecules are linked into simple C(10) chains by N—H⋯O hydrogen bonds. In each of the salts, i.e., C₁₁H₁₇N₂O⁺·C₇H₃N₂O₆⁻, (II), C₁₁H₁₇N₂O⁺·C₆H₂N₃O₇⁻, (III), and C₁₁H₁₇N₂O⁺·C₇H₆NO₂⁻·H₂O, (IV), the effectively planar 4-meth­oxy­phenyl substituent again occupies an equatorial site on the piperazine ring. In (II), two of the nitro groups are disordered over two sets of atomic sites and the bond distances in the anion indicate considerable delocalization of the negative charge over the C atoms of the ring. The ions in (II) are linked by two N—H⋯O hydrogen bonds to form a cyclic, centrosymmetric four-ion aggregate; those in (III) are linked by a combination of N—H⋯O and C—H⋯π(arene) hydrogen bonds to form sheets; and the components of (IV) are linked by N—H⋯O, O—H⋯O and C—H⋯π(arene) hydrogen bonds to form a three-dimensional framework structure. Comparisons are made with the structures of some related compounds.