The two isomeric compounds 4-amino-ONN-azoxybenzene [or 1-(4-aminophenyl)-2-phenyldiazene 2-oxide], i.e. the alpha isomer, and 4-amino-NNO-azoxybenzene [or 2-(4-aminophenyl)-1-phenyldiazene 2-oxide], i.e. the beta isomer, both C(12)H(11)N(3)O, crystallized from a polar solvent in orthorhombic space groups, and their crystal and molecular structures have been determined using X-ray diffraction. There are no significant differences in the bond lengths and valence angles in the two isomers, in comparison with their monoclinic polymorphs. However, the conformations of the molecules are different due to rotation along the Ar-N bonds. In the alpha isomer, the benzene rings are twisted by 31.5 (2) and 14.4 (2) degrees towards the plane of the azoxy group; the torsion angles along the Ar-N bond in the beta isomer are 24.3 (3) and 23.5 (3) degrees. Quantum-mechanical calculations indicate that planar conformations are energetically favourable for both isomers. The N-H.O hydrogen bonds observed in both networks may be responsible for the deformation of these flexible molecules.