The UV-light-induced CO release characteristics of a series of ruthenium(II) carbonyl complexes of the form trans-Cl[RuLCl2(CO)(2)] (L = 4,4'-dimethyl-2,2'-bipyridine, 4'-methyl-2,2'-bipyridine-4-carboxylic acid, or 2,2'-bipyridine-4,4'-dicarboxylic acid) have been elucidated using a combination of UV-vis absorbance and Fourier transform infrared spectroscopies, multivariate curve resolution alternating least-squares analysis, and density functional theory calculations. In acetonitrile, photolysis appears to proceed via a serial three-step mechanism involving the sequential formation of [RuL(CO)(CH3CN)Cl-2], [RuL(CH3CN)(2)Cl-2], and [RuL(CH3CN)(3)Cl](+). Release of the first CO molecule occurs quickly (k1 3 min(-1)), while release of the second CO molecule proceeds at a much more modest rate (k(2) = 0.099-0.17 min(-1)) and is slowed by the presence of electron-withdrawing carboxyl substituents on the bipyridine ligand. In aqueous media (1% dimethyl sulfoxide in H2O), the two photodecarbonylation steps proceed much more slowly (k(1) = 0.46-1.3 min-1 and k(2) = 0.026-0.035 min(-1), respectively) and the influence of the carboxyl groups is less pronounced. These results have implications for the design of new light-responsive CO-releasing molecules ("photoCORMs") intended for future medical use.