The synthesis, spectroscopic characterization and crystal structure of [(DMSO)(2)H][trans-RuCl4(DMSO)(CO)] (3) and mer,cis-RuCl3(DMSO)(2)(CO) (4) are reported (DMSO = O-bonded dimethyl sulfoxide). The two complexes are easily synthesized from [(DMSO)(2)H][trans-RuCl4(DMSO)(2)] (1) and mer,trans-RuCl3(DMSO)(2)(DMSO) (2), respectively, upon reaction with carbon monoxide at room temperature and atmospheric pressure. They represent the first examples of Ru(III) chloride-DMSO-carbonyl complexes. Coordination of carbon monoxide induces the S to O linkage isomerization of the DMSO ligand trans to it. [(DMSO)(2)H][trans-RuCl4(DMSO)(CO)] : orthorhombic, space group Pnma, Z = 4, a = 10.564(1) Angstrom, b = 14.620(3) Angstrom, c = 12.312(1) Angstrom. mer,cis-RuCl3(DMSO)(2)(CO) : triclinic, space group P1, Z = 4, a = 6.991(9) Angstrom, b = 13.98(1) Angstrom, c = 14.86(2) Angstrom, alpha = 82.76(8)degrees, beta = 85.83(8)degrees, gamma = 75.41(9)degrees. In both 3 and 4 the DMSO ligand trans to CO is weakly bonded and easily replaced by a nitrogen donor ligand such as NH3 or pyridine.