2,2'-({4-[(4-Nitro-phen-yl)diazen-yl]phen-yl}imino)-diethanol.

In the title compound, C16H18N4O4, the mol-ecule assumes an E conformation with respect to the N=N double bond. The aromatic rings are not coplanar, with a dihedral angle of 7.51 (8)°. The nitro group is tilted by 4.71 (11)° relative to the attached benzene ring. In the crystal, mol-ecules are connected through O-H⋯O hydrogen bonds forming a double-stranded chain parallel to the b axis.