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Hydrogenolysis reactions of cobalt acyl complexes, [Co(CO)3(L)(COR)] (L = phosphine, R = Me, n Pr) have been monitored using in situ IR spectroscopy at moderate temperatures (<75 °C) and pressures (<25 bar). The reactions provide a model for the product-formation step in phosphine-modified, cobalt-catalyzed hydroformylation. The reaction kinetics are dependent on L, with the fastest rate being observed for the complex containing n-pentyl-9-phosphabicyclo[4.2.1]nonane (a 5-PhobPC5). The observed dependence of rate on H2 and CO pressure is consistent with a mechanism involving initial CO dissociation, followed by reaction of [Co(CO)2(L)(COR)] with H2. Isotopic exchange experiments, monitored by IR spectroscopy, demonstrate that both terminal and acetyl carbonyls of [Co(CO)3(L)(COR)] exchange with free 13CO. Kinetic data are also reported for reactions of [Co(CO)3(L)(COR)] with triphenyltinhydride. A zero-order dependence on [Ph3SnH] (at large excess) and positive values of ΔS ‡ demonstrate rate-determining CO dissociation. For a series of less bulky, symmetrical phosphines, the rates follow the sequence PEt3 < PMe2Ph ∼ PEtPh2 < PPh3 < P(4-ClC6H4)3, in accord with their electron-donating strength. Higher rates are found for more bulky phosphines, and the fastest rate is again found for L = a 5-PhobPC5. Calculations using density functional theory indicate that the CO dissociation energy for [Co(CO)3(L)(COMe)] is influenced by the stereoelectronic properties of L, with steric bulk having a substantial effect. X-ray crystal structures are reported for [Co(CO)3(PEtPh2)(COMe)], [Co(CO)3(s-PhobPC5)]2, and [Co(CO)3(a 5-PhobPC5)]2.