A few ruthenium based metal carbonyl complexes, e.g. CORM-2 and CORM-3, have therapeutic activity attributed to their ability to deliver CO to biological targets. In this work, a series of related complexes with the formula Ru(CO) 3 Cl 2 L (L = DMSO ( 3 ), l -H 3 CSO(CH 2 ) 2 CH(NH 2 )CO 2 H) ( 6a ); d , l -H 3 CSO(CH 2 ) 2 CH(NH 2 )CO 2 H ( 6b ); 3-NC 5 H 4 (CH 2 ) 2 SO 3 Na ( 7 ); 4-NC 5 H 4 (CH 2 ) 2 SO 3 Na ( 8 ); PTA ( 9 ); DAPTA ( 10 ); H 3 CS(CH 2 ) 2 CH(OH)CO 2 H ( 11 ); CNCMe 2 CO 2 Me ( 12 ); CNCMeEtCO 2 Me ( 13 ); CN( c -C 3 H 4 )CO 2 Et) ( 14 )) were designed, synthesized and studied. The effects of L on their stability, CO release profile, cytotoxicity and anti-inflammatory properties are described. The stability in aqueous solution depends on the nature of L as shown using HPLC and LC-MS studies. The isocyanide derivatives are the least stable complexes, and the S-bound methionine oxide derivative is the more stable one. The complexes do not release CO gas to the headspace, but release CO 2 instead. X-ray diffraction of crystals of the model protein Hen Egg White Lysozyme soaked with 6b (4UWN) and 8 (4UWV) shows the addition of Ru II (CO)(H 2 O) 4 at the His15 binding site. Soakings with 7 (4UWU) produced the metallacarboxylate Ru(COOH)(CO)(H 2 O) 3 + bound to the His15 site. The aqueous chemistry of these complexes is governed by the water–gas shift reaction initiated with the nucleophilic attack of HO − on coordinated CO. DFT calculations show this addition to be essentially barrierless. The complexes have low cytotoxicity and low hemolytic indices. Following i.v. administration of CORM-3, the in vivo bio-distribution of CO differs from that obtained with CO inhalation or with heme oxygenase stimulation. A mechanism for CO transport and delivery from these complexes is proposed.