(E)-4-Hy-droxy-2-[(2-hy-droxy-phen-yl)iminiometh-yl]phenolate.

The title compound, C(13)H(11)NO(3), crystallizes in a zwitterionic form and has a trans configuration about the C=N bond. The mol-ecule is almost planar, the dihedral angle between the two benzene rings being 4.32 (8)°. The two hy-droxy substit-uents are coplanar with each of their attached benzene rings [r.m.s. deviations of 0.0053 (2) and 0.0052 (2) Å]. An intra-molecular N-H⋯O hydrogen bond formed between the iminium N and the phenolate O atom generates an S(6) ring motif. In the crystal, the mol-ecules are linked through O-H⋯O hydrogen bonds into chains along [110]. Two neighbouring chains are further connected through O-H⋯O hydrogen bonds in an anti-parallel manner. π-π inter-actions are also observed, with centroid-centroid distances of 3.7115 (19) and 3.743 (2) Å.