Crystals of the title compound are monoclinic, a= 8.939(3), b= 18.045(7), c= 11.363(3)Å, β= 91.52(2)°, Z= 4, space group P21/n. The structure was determined by Patterson and Fourier syntheses and refined by full-matrix least-squares procedures to a final R of 0.041 for 2 720 independent reflections measured by diffractometer. The co-ordination geometry about the ruthenium atom is essentially octahedral with cis-chlorine atoms. Of the four dimethyl sulphoxide ligands three are S- and one is O-bonded, the O-bonded ligand being trans to a S-bonded ligand. Important mean bond distances are : Ru–Cl 2.435(1), Ru–S 2.277(1)(trans to Cl), 2.252(1)(trans to O), Ru–O 2.142(3), S–O 1.484(5)(S-bonded), and 1.557(4)Å(O-bonded).