The crystal structure of bis(cyclotetramethylene)diphosphine disulphide,S has been determined from three-dimensional X-ray diffraction data. The unit cell is triclinic with space group PI (number 2), dimensions a = 7.65, b = 6.90, c = 5.88/~; e = 75 ° 30', p= 104 ° 15', ~, = 92 ° 36', and contains one molecule. The structure was refined by Fourier and full-matrix least-squares methods on 771 independent observed reflexions to R= 10"4%. The molecule possesses a centre of symmetry at the mid-point between the two phosphorus atoms and the two sulphur atoms are trans to each other. The environment of each phosphorus atom is that of a distorted tetrahedron; the inclusion of phosphorus in a ring system reduces the angle C-P-C from tetrahedral to 96"6 °. The ring system is saturated and consequently puckered.