The preparation and structural characterization of three cationic Ru(II)-dmso carbonyls and of four neutral mono- and dicarbonyl Os(II)-dmso derivatives is reported. The two monocarbonyl species fac-[Ru(CO)(dmso-O)3(dmso-S)2][PF6]2 (11) and cis,cis,cis-[RuCl(CO)(dmso-O)2(dmso-S)2][PF6] (12) were obtained from the neutral monocarbonyl precursor cis,trans,cis-[RuCl2(CO)(dmso-O)(dmso-S)2] (3) upon stepwise replacement of the chlorides with dmso, that binds in each case through the oxygen atom. The dicarbonyl cationic complex cis,cis,trans-[Ru(CO)2(dmso-O)2(dmso-S)Cl][PF6] (13) was instead obtained upon treatment of the neutral tricarbonyl precursor fac-[RuCl2(CO)3(dmso-O)] (8) with AgPF6 in the presence of DMSO: replacement of a Cl(-) with a dmso-O implied also the substitution of one CO ligand by another dmso (that binds through S trans to Cl). The Os(II) carbonyls trans,trans,trans-[OsCl2(CO)(dmso-O)(dmso-S)2] (17), trans,cis,cis-[OsCl2(CO)2(dmso-O)2] (18), cis,mer-[OsCl2(CO)(dmso-S)3] (19), and cis,trans,cis-[OsCl2(CO)(dmso-O)(dmso-S)2] (20) were obtained by treatment of the Os(II)-dmso precursors trans-[OsCl2(dmso-S)4] (14) and cis,fac-[OsCl2(dmso-O)(dmso-S)3] (15) with CO. Each one of them is structurally similar to an already known Ru(II) analog, even though--in agreement with the expected greater inertness of Os(II)--more forcing reaction conditions were required for their preparation. Interestingly, compound 20 could not be isolated in pure form, but only as a 1:1 cocrystallized mixture with its precursor 15. The dmso ligand is always bound through the oxygen atom when trans to CO. We are confident that the new Ru(II)- and Os(II)-dmso carbonyl species described here represent a contribution to expand the pool of complexes bearing some easily replaceable dmso ligands to be used as well-behaved precursors in inorganic synthesis.