The reaction of 4-hy-droxy-pyridine and oleum produces 4-hy-droxy-pyridinium-3-sulfonate, C(5)H(5)NO(4)S, which shows delocalized bonds in the six-membered ring. In the crystal, adjacent zwitterions are linked by N-H⋯O and O-H⋯O hydrogen bonds into a layer motif. The crystal studied was a racemic twin.
