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The asymmetric unit of the title cystosinium salt derivative, 2C 4 H 6 N 3 O + Á-C4H4O62-\cdot-ÁH 2 O, contains two cytosinium cations, one tartaric acid anion and a water molecule. The two cytosinium cations are almost planar (r.m.s. deviations of the fitted atoms are 0.0151 and 0.0213 Å ). The crystal structure features C-HÁ Á ÁO, N-HÁ Á ÁO and O-HÁ Á ÁO interactions. Further C-OÁ Á Á and -interactions are observed along the ab plane, contributing to the crystal stability. Structure descriptionPyrimidine-based derivatives have attracted a great deal of attention in terms of their hydrogen-bonding patterns. Cytosinium is one of the naturally occurring base molecules found in DNA and RNA (Portalone et al., 2009 (Sridhar & Ravikumar, 2011). As part of our investigations on the growth and characterization of semi-organic crystals containing the nucleic acid component cytosine, we report herein the crystal structure determination and the geometry optimization of the title compound.A perspective view of the title compound with the atomic numbering scheme is illustrated in Fig. 1 The crystal structure features C-HÁ Á ÁO, N-HÁ Á ÁO and O-HÁ Á ÁO interactions. The interaction between N4 and O8 through H4A, N6 and O7 through H6B and N1 and O3 through H1A, N3 and O4 through H3A occur alternately as chain links and form a three-dimensional network enclosing R 2 2 (8) ring motifs. The interaction between N3 and O7 through H3B and N6 and O6 through H6C form parallel chains along the a-and c-axis directions, respectively (Fig. 2). Similarly the interactions between N2 and O3 through H2A and N5 and O8 through H5A form infinite parallel chains along the c-and aaxis directions, respectively. The O6-H6AÁ Á ÁO9 interaction encloses parallel chains along the ab plane (Fig. 3). Also the interactions of C4, C11 with O1 through H4, H11 form infinite parallel chains along the b-and c-axis directions, respectively (Fig. 4). The O9 interactions with O2, O5 through H9A, H9B Figure 2