The structures of diastereomeric pairs consisting of (S)- and (R)-2-methylpiperazine with (2S,3S)-tartaric acid are both 1:1 salts, namely (S)-2-methylpiperazinium (2S,3S)-tartrate dihydrate, C(5)H(14)N(2)(2+).C(4)H(4)O(6)(2-).2H(2)O, (I), and (R)-2-methylpiperazinium (2S,3S)-tartrate dihydrate, C(5)H(14)N(2)(2+).C(4)H(4)O(6)(2-).2H(2)O, (II), which reveal the formation of well defined ammonium carboxylate salts linked via strong intermolecular hydrogen bonds. Unlike the situation in the more soluble salt (II), the alternating columns of tartrate and ammonium ions of the less soluble salt (I) are packed neatly in a grid around the a axis, which incorporates water molecules at regular intervals. The increased efficiency of packing for (I) is evident in its lower 'packing coefficient', and the hydrogen-bond contribution is stronger in the more soluble salt (II).