The title molecular complex, C 5 H 4 N 2 O 3 .C 7 H 6 O 3 , is hydrogen bonded, the molecules being linked by an O-H...O hydrogen bond between the phenol hydroxyl group and the N-oxide O atom [O…O 2.642 (2) A]. The 3-hydroxybenzoic acid molecules form hydrogen-bonded pairs across centres of symmetry in the complex, with an O...O distance of 2.628(2)A. The interplanar angle between the aromatic rings in the two components is 63.86(6)°.