CORM-3, fac-Ru(CO)3Cl(κ2-H2NCH2CO2), is a well-known carbon monoxide releasing molecule (CORM) capable of delivering CO in vivo. Herein we show for the first time that the interactions of CORM-3 with proteins result in the loss of a chloride ion, glycinate, and one CO ligand. The rapid formation of stable adducts between the protein and the remaining cis-RuII(CO)2 fragments was confirmed by Inductively Coupled Plasma-Atomic Emission Spectrocopy (ICP-AES), Liquid-Chromatography Mass Spectrometry (LC-MS), Infrared Spectroscopy (IR), and X-ray crystallography. Three Ru coordination sites are observed in the structure of hen egg white lysozyme crystals soaked with CORM-3. The site with highest Ru occupancy (80%) shows a fac-(His15)Ru(CO)2(H2O)3 structure.