2,3,5,6-Tetraoxopiperazine (I), ammonium 2,3,5,6-tetraoxo-4-nitropyridinate crystal hydrate (II), co-crystallized dimethylammonium 2,3,5,6-tetraoxo-4- nitropyridinate and dimethylammonium chloride (III), and ammonium 2,6-dioxo-3,5-dinitropyridinate (IV) were isolated in the crystalline state and studied by X-ray diffraction. All heterocycles are planar and nonaromatic. They consist of two conjugated moieties linked together by C-C single bonds. The short C.O contacts (2.841-3.048 Å), which link the molecules to form zigzag two-dimensional chains, make a considerable contribution to the formation of the crystal structure of I, whereas hydrogen bonds that form three-dimensional networks play the most important role in the formation of the crystals of II-IV. © 2014 Pleiades Publishing, Inc.