2-Hy-droxy-5-{[(E)-4-meth-oxy-benzyl-idene]aza-nium-yl}benzoate.

In the title zwitterion, C(15)H(13)NO(4), obtained from the condensation of 5-amino-salicylic acid and 4-meth-oxy-benz-alde-hyde, the 4-hydoxyanilinic group of the 5-amino-salicylic acid moiety and the 4-meth-oxy-benzaldehyde moiety are twisted with respect to one another, making a dihedral angle of 10.37 (7)°. The carboxyl-ate group makes a dihedral angle of 5.7 (2)° with the parent 4-hydoxyanilinic group. An intra-molecular O-H⋯O hydrogen bond forms an S(6) ring motif. In the crystal, inter-molecular C-H⋯O and N-H⋯O hydrogen bonds with R(2) (1)(7) ring motifs link the mol-ecules into infinite chains extending along the c axis. The occurence of slipped π-π stacking between symmetry-related aromatic rings reinforces the packing.