The crystal structures of a pair of diastereomeric 1:2 salts of (R)- and (S)-2-methylpiperazine with (2S,3S)-tartaric acid, namely (R)-2-methylpiperazinediium bis[hydrogen (2S,3S)-tartrate] monohydrate, (I), and (S)-2-methylpiperazinediium bis[hydrogen (2S,3S)-tartrate] monohydrate, (II), both C(5)H(14)N(2)(2+).2C(4)H(5)O(6)(-).H(2)O, each reveal the formation of well-defined head-to-tail-connected hydrogen tartrate chains; these chains are linked into a two-dimensional sheet via intermolecular hydrogen bonds involving hydroxy groups and water molecules, resulting in a layer structure. The (R)-2-methylpiperazinediium ions lie between the hydrogen tartrate layers in the most stable equatorial conformation in (I), whereas in (II), these ions are in an unstable axial position inside the more interconnected layers and form a larger number of intermolecular hydrogen bonds than are observed in (I).