The crystal structures of cis- and trans-dichlorostilbene were determined from X-ray diffraction data, cw-dichlorostilbene crystallizes in the rhombohedral space group R3̄ with 18 formula units per unit cell (821 observed independent reflexions, R = 0.064) with the dimensions a = b = 3323, c = 601 pm, γ = 120°. The molecule corresponds to the point group C1 with a C = C bond length of 133 pm; the dihedral angles of the phenyl rings with the corresponding C = C-Cl plane are 48° and 72°, respectively. Trans-dichlorostilbene crystallizes in the monoclinic space group P21/n with two formula units per unit cell (1138 independent reflexions, R = 0.056) with the dimensions a = 572, b = 1733, c = 641 pm, β = 111°. The molecule is centrosymmetric (Ci) with a C = C bond length of 133 pm; the dihedral angle of the phenyl rings and the Cl - C = C plane is 71°. The stilbene molecules are disordered about the centre of symmetry in two orientations with the ratio 3:7.