The crystal and molecular structure of 2,5-dithio-l-chloro-l-thiophosphorus(V) cyclopentane S II P // CH2 ~CH2 has been determined from X-ray data by the direct method of symbolic addition and has been refined by three-dimensional full-matrix least-squares methods. The unit cell is orthorhombic with space group Pbca (D~ no. 61), dimensions of a= 11.095, b= 10.288, c= 12= 739 ~, and contains 8 molecules. The final R value based on 940 independent observed reflexions was 8"7 %. The phosphorus atom is approximately tetrahedrally surrounded by three sulphur atoms and one chlorine atom. The two heterocyclic sulphur atoms both form P-S bonds of 2.051 + 0.004 ~ and + 0.005 ,~ respectively. These are normal single bonds. The phosphorus atom forms a double bond of length 1.910+ 0.005 ~ to the third sulphur atom, and a single bond to chlorine of length 2.059 + 0.004 A. Two factors which distort the tetrahedral arrangement are the P=S bond and the inclusion of phosphorus in a five-membered heterocyclic ring. The heterocyclic ring is strained, with angles S-P-S = 100-5 °, P-S-C = 94-1, and 98-0 ° and S-C-C = 111.5 and 113.7 °. In one molecule each chlorine atom receives two hydrogen bonds from other molecules, and two of the methylene hydrogen atoms are hydrogen bonded to chlorine atoms in other molecules. There is some further evidence that Pauling's value for the van der Waals radius of sulphur of 1.85 .~ is too large, and a value of 1.72-1.73 A is suggested.