Time-resolved infrared spectroscopic studies have been used to characterize the reactive intermediate CH3C(O)Co(CO)(2)-PPh3 (I-Co), which is relevant to the mechanism of the catalysis of alkene hydroformylation by the phosphine-modified cobalt carbonyls. Step-scan FTIR and (variable) single-frequency time-resolved infrared detection on the microsecond time scale were used to record the spectrum of I-Co and to demonstrate that the principal photoproduct of the subsequent reaction of this species at P-CO = 1 atm is the methyl cobalt complex CH3Co-(CO)(3)PPh3 (M-Co). At higher P-CO the trapping of I-Co with CO to re-form CH3C(O)Co(CO)(3)PPh3 (A(Co)) (rate = k(CO)[CO][I-Co]) was shown to become competitive with the rate of acetyl-to-cobalt methyl migration to give M-Co (rate = k(M)[I-Co]) Activation parameters for the competing pathways in benzene were determined to be Delta H-CO(double dagger) = 5.7 +/- 0.4 kJ mol(-1), Delta S-CO(double dagger) = -91 +/- 12 J mol(-1) K-1 and Delta H-M(double dagger) = 40 +/- 2 kJ mol(-1), Delta S-M(double dagger) = -19 +/- 5 J mol(-1) K-1. The effects of varying the solvent on the competitive reactions of I-Co were also explored, and the mechanistic implications of these results are discussed.