Short-wavelength (308 nm) photoinduced reactions of binary ruthenium carbonyls in the presence of varying concentrations of carbon monoxide are investigated by means of flash photolysis (XeCl excimer laser) in combination with time-resolved IR spectroscopy. Photodissociation of CO from Ru-3(CO)(12)l yields the Ru-3(CO)(11) fragment in the CO-bridged form. Beyond the known re-formation of the parent complex, this species under CO atmosphere takes up a total number of three CO ligands to form a 1:1 mixture of Ru(CO)S and Ru-2(CO)(9) as the ultimately observed products. This process involves the intermediate formation of Ru-3(CO)(12)(mu-CO), which decays by first-order kinetics with a rate constant of about 2.2 x 10(4) s(-1) at ambient temperature. Complementary studies with Ru(CO)(5) and (in situ generated) Ru-2(CO)(9) as starting materials provide insight into the mechanism by which Ru(CO)(5) is converted into Ru-3(CO)(12) and/or Ru-2(CO)(9), depending on the CO concentration and the photolysis conditions. The solvated Ru(CO)4 fragment, the dinuclear Ru-2(CO)(8), and Ru-3(CO)(12)(mu-CO) are observed as transient species in addition to Ru-2(CO)(9), which is stable on the time scale of these experiments.