A proton-transfer compound, 1-phenylbiguanidium 5-nitro-2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-olate monohydrate, C(8)H(12)N(5)(+) x C(4)H(2)N(3)O(5)(-) x H(2)O, has been synthesized by a reaction between dilituric acid (5-nitro-2,4,6-trihydroxypyrimidine, Dilit) and phenylbiguanide (N-phenylimidocarbonimidic diamide, Big). This compound cocrystallized as a 1:1 adduct, and the asymmetric unit consists of two dilituric amino-oxo planar tautomeric anions (Dilit(-)), two monoprotonated phenylbiguanidium cations (BigH(+)) and two water molecules of crystallization (Z' = 2). Protonation occurs at the N atom attached to the phenyl ring of Big as a result of the proton-transfer process from the acidic hydroxy group of Dilit. In the crystal structure, the hydrated 1:1 adduct is stabilized by 25 two- and three-center hydrogen bonds.