A new crystalline form of benzene-1,2-diamine, C(6)H(8)N(2), crystallizing in the space group Pbca, has been identified during screening for cocrystals. The crystals are constructed from molecular bilayers parallel to (001) that have the polar amino groups directed to the inside and the aromatic groups, showing a herringbone arrangement, directed to the outside. The known monoclinic form and the new orthorhombic polymorph exhibit two-dimensional isostructurality as the crystals consist of nearly identical bilayers. In the monoclinic form, neighbouring bilayers are generated by a unit translation along the a axis, whereas in the orthorhombic form they are generated by a c-glide. Moreover, the new form of benzene-1,2-diamine is essentially isomorphous with the only known form of 2-aminophenol.