Two related proton-transfer compounds, namely piperazine-1,4-diium 4-oxo-4H-pyran-2,6-dicarboxylate monohydrate, C(4)H(12)N(2)(2+)·C(7)H(2)O(6)(2-)·H(2)O or (pipzH(2))(cdo)·H(2)O, (I), and piperazine-1,4-diium bis(6-carboxy-4-oxo-4H-pyran-2-carboxylate), C(4)H(12)N(2)(2+)·2C(7)H(3)O(6)(-) or (pipzH(2))(cdoH)(2), (II), were obtained by the reaction of 4-oxo-4H-pyran-2,6-dicarboxylic acid (chelidonic acid, cdoH(2)) and piperazine (pipz). In (I), both carboxyl H atoms of chelidonic acid have been transferred to piperazine to form the piperazine-1,4-diium ion. The structure is a monohydrate. All potential N-H donors are involved in N-H···O hydrogen bonds. The water molecule spans two anions via the 4-oxo group of the pyranose ring and a carboxylate O atom. The hydrogen-bonding motif is essentially two-dimensional. The structure is a pseudomerohedral twin. In the asymmetric unit of (II), the anion consists of monodeprotonated chelidonic acid, while the piperazine-1,4-diium cation is located on an inversion centre. The single carboxyl H atom is disordered in two respects. Firstly, the disordered H atom is shared equally by both carboxylic acid groups. Secondly, the H atom is statistically disordered between two positions on either side of a centre of symmetry and is engaged in a very short hydrogen-bonding interaction; the relevant O···O distances are 2.4549 (11) and 2.4395 (11) Å, and the O-H···O angles are 177 (6) and 177 (5)°, respectively. Further hydrogen bonding of the type N-H···O places the (pipzH(2))(2+) cations in pockets formed by the chains of (cdoH)(-) anions. In contrast with (I), the (pipzH(2))(2+) cations form hydrogen-bonding arrays that are perpendicular to the anions, yielding a three-dimensional hydrogen-bonding motif. The structures of both (I) and (II) also feature π-π stacking interactions between aromatic rings.