4-(4-Nitro-benz-yl)pyridinium 3-carb-oxy-4-hy-droxy-benzene-sulfonate.

In the title salt, C(12)H(11)N(2)O(2) (+)·C(7)H(5)O(6)S(-), the dihedral angle between the benzene and pyridine rings in the 4-(4-nitro-benz-yl)pyridinium cation is 82.7 (2)°. Within the anion there is an intramolecular hydroxy-O-H⋯O(carboxylic acid) bond. In the crystal, the cation forms a single N(+)-H⋯O(sulfonate) hydrogen bond with the anion. These cation-anion pairs inter-act through duplex anion carb-oxy-lic acid O-H⋯O(sulfonate) hydrogen bonds, giving a centrosymmetric cyclic association [graph set R(2) (2)(16)]. The crystals studied were non-merohedrally twinned.