The reactivity of cis- and trans-RuCl2(DMSO)(4) with carbon monoxide has been thoroughly investigated. Depending on the choice of the solvent and the reaction conditions, cis-RuCl2(DMSO)(4) reacts with CO at ambient pressure replacing one, two or three DMSO molecules. The following derivatives have been isolated : trans,cis,cis-Ru-(CO)(DMSO)(DMSO)(2)Cl-2 (1), cis,cis,cis-Ru(CO)(2)(DMSO)(DMSO)Cl-2 (2), cis,trans,cis-Ru(CO)(2)(DMSO)(2)Cl-2 (3) and fac-Ru(CO)(3)(DMSO)Cl-2 (4). Compounds 1, 3 and 4 have been structurally characterized. 1 . 0.5H(2)O : monoclinic, P2(1)/c, Z = 8, a = 15.408(1) Angstrom, b = 15.398(2) Angstrom, c = 14.826(1) Angstrom, beta = 107.46(1)degrees. 3 : orthorhombic, Pca2(1), Z = 8, a = 15.853(3) Angstrom, b = 11.380(2) Angstrom, c = 14.874(2) Angstrom. 4 : orthorhombic, P2(1)2(1)2(1), Z = 4, a = 7.300(2) Angstrom, b = 9.297(2) Angstrom, c = 16.515(4) Angstrom, beta = 107.46(1)degrees. Reaction of trans-RuCl2(DMSO)(4) with CO at room temperature and ambient pressure leads to the replacement of either one or two DMSO ligands, depending on the reaction time, yielding trans,trans,trans-Ru(CO)(DMSO)(DMSO)(2)Cl-2 (5) and cis,cis,trans-Ru(CO)(2)(DMSO)(2)Cl-2 (6). The crystal structure of 5 has been determined. 5 : monoclinic, P2(1), Z = 4, a = 7.417(3) Angstrom, b = 26.808(6) Angstrom, c = 8.368(4) Angstrom, beta = 107.55(7)degrees. Complex 3 is the thermodynamically most stable bis(carbonyl) species. Compounds 1-6 represent the first example of well-characterized Ru(II) chloride-DMSO-carbonyl complexes. Their most striking common feature is that coordination of carbon monoxide induces the selective isomerization of the DMSO trans to it from S- to O-bonding. Compounds 1-6 are versatile precursors for the synthesis of carbonylated Ru(II) species in which the DMSO ligands are partially or completely replaced by other ligands. Reaction of 1-6 with pyridine has been investigated and several products were isolated.