Bis(4-hydroxy-pyridinium) sulfate monohydrate.

In the crystal structure of the title salt, 2C(5)H(6)NO(+)·SO(4) (2-)·H(2)O, one planar (r.m.s. deviation = 0.01 Å) cation is stacked approximately over the other [dihedral angle between planes = 8.6 (1)°]. The pyridinium and hydr-oxy H atoms are hydrogen-bond donor atoms to the O atoms of the sulfate anion; the cations, anions and water mol-ecules are consolidated into a three-dimensional network through O-H⋯O and N-H⋯O hydrogen bonds.