The crystal structure of 2,2'-diaminodiphenyl disulphide has been determined accurately from threedimensional X-ray diffraction data. Crystals are orthorhombic, and belong to the space group Pbca. The unit cell has dimensions a=8-21, b= 13"14, c=22"77/~,, and contains eight molecules. The positional and anisotropic thermal parameters for the sulphur, carbon and nitrogen atoms have been refined by Fourier and full-matrix least-squares methods on 1313 independent observed reflexions. In the later stages hydrogen atoms were included in fixed positions, and the final agreement factor R = 8"6%. The central part of the molecule C-S-S-C has a skewed non-planar conformation similar to H202, with a dihedral angle of 90.5 °. The S-S bond length is 2.06/~, which is longer than in diphenyl and dibenzyl disulphides. The C-S bond lengths of 1.77 and 1 "75/~ are significantly short. This indicates some double bond character, which may be explained by pzr-dzt bonding. The benzene rings are inclined at an angle of 32"5 ° to each other. ExperimentalA sample of 2,2'-diaminodiphenyl disulphide, H2N-C6H4-S-S-C6H4-NH2, kindly provided by the Clayton Aniline Co. Ltd, was recrystallized from ethanol and gave pale yellow-green platelets elongated along a. These showed straight extinction parallel and perpendicular to a when viewed under the polarizing microscope. The density of the crystals was measured by flotation in aqueous potassium iodide solution, and was found to be 1.33 (5) g.cm -3.Considerable difficulty was experienced in selecting a single crystal for X-ray examination. Many crystals consisted of several parallel platelets, indistinguishable from a single crystal under the polarizing microscope but detectable on higher layer line Weissenberg photographs. Eventually a suitable single crystal of dimensions 0.4 x 0.3 x 0.15 mm was used to obtain oscillation photographs about the a and b axes, and equi-inclination Weissenberg photographs for the Okl-6kl and hOl zones. The relative intensities of 1313 independent reflexions were measured visually and corrected for Lorentz and polarization factors. Absorption corrections were considered unnecessary, in view of the small crystal size compared with the optimum size of 0.50 mm (Buerger, 1958). Reftexions too weak to be observed were omitted. Crystal data C12H12N282