We have performed X-ray diffraction measurements on single crystals of Na-3[V10O28](C4N3OH5)(3)-(C4N3OH6)(3)center dot 10H(2)O as a function of the temperature. When the sample is cooled, from room temperature to 100 K, we have observed additional peaks well indexed in PI, while the phase at room temperature crystallizes in P (1) over bar. The molecular structure at 210 K indicates that the center of inversion is located between two cytosinium molecules, formally described with a charge of +0.5. When this crystal is heated to room temperature and the structure in P (1) over bar reindexed, some peaks remained unindexed. A protonation-deprotonation process gives rise to additional diffraction peaks at temperatures lower than 210 K. The triply bridged hydrogen bonded cytosine-cylosinum dimer is discussed according to the results of the charge density analysis and topological analysis at 210 K. The structure at 100 K has been completely solved based on a comparative study with other compounds containing cytosine-cytosinium dimer. This description could be considered as a reference for such dimer. It could help for discrimination between cytosine and cytosinium molecules, for any new structure containing a cytosine-cytosinium pair, and for which the quality does not allow a precise determination of the hydrogen localization.