Reaction of the mixed-metal tetranuclear clusters H2FeRu3(CO)13, H2FeRu2Os(CO)13, H2FeRuOs2(CO)13, HCoRu3(CO)13, H2FeRu3(CO)12L (L = PMe2Ph, PPh3), [HFeRu3(CO)13], and [CoRu3(CO)13]- with CO under relatively mild conditions (P(CO) = 1 atm, t = 0-80 °C) results in their fragmentation to give trimeric and monomeric products. The fragmentation reactions are generally quite selective: H2FeRu2Os(CO)13, for example, reacts with CO to give Ru2Os(CO)12, Fe(CO)5, and H2 with no RuOs2(CO)12, FeRu2(CO)12, or FeRuOs(CO)12 detected in the product mixture. This reaction and the corresponding fragmentation of H2FeRuOs2(CO)13 provide a convenient means of separating and isolating the mixed-metal trimers Ru2Os(CO)12 and RuOs2(CO)12. The reactivity of a particular cluster toward CO is greatly dependent on its metal composition and/or its structure. For example, H2FeRu2Os(CO)13 reacts slowly over a period of several days while the breakdown of HCoRu3(CO)13 is complete within 1 h. The tetrahydride clusters H4FeRu3(CO)12 and H4Ru4(CO)12 first react with CO to displace H2 and form the corresponding dihydride clusters which subsequently fragment under the reaction conditions. Kinetic measurements indicate that the fragmentation of H2FeRu3(CO)13 to produce Ru3(CO)12, Fe(CO)s, and H2 proceeds largely by a second-order, [CO]-dependent path with activation parameters ΔH°* = 20.0 ± 2.0 kcal/mol and ΔS°* = -25.4 ± 5.8 cal/(mol K). Likewise, fragmentation of H2Ru4(CO)13 to form Ru3(CO)12, Ru(CO)5, and H2 proceeds via a second-order, [CO]-dependent path with ΔH°* = 12.5 ± 0.5 kcal/mol and ΔS°* = -36.6 ± 1.6 cal/(mol K).