(E)-4-[2-(2-Hy-droxy-benzo-yl)-hydra-zin-ylidene]penta-noic acid.

The title mol-ecule, C(12)H(14)N(2)O(4), adopts a trans configuration with respect to the C=N double bond. The amino group is involved in an intra-molecular N-H⋯O hydrogen bond. In the crystal structure, inter-molecular O-H⋯O hydrogen bonds link the mol-ecules into doubled sheets parallel to the (101) plane.