Abstract Radiative relaxation of Cr(CO) 5 − was investigated by two techniques: a standard two-pulse photodissociation experiment and by using the branching ratio of its reaction with oxygen as an ion thermometric probe. Photoexcitation at 1064 nm was used to prepare highly vibrationally excited Cr(CO) 5 −. Although the overall oxidation rate changes only slightly upon excitation (actually decreasing by a factor of 1.2 ± 0.1), the primary product distribution shifts dramatically, from Cr(CO) 3O − (the thermodynamic product) to Cr(CO) 3O 2 − (the kinetic product). The two-pulse photodissociation measurement gave a radiative relaxation rate constant ( k rad) of 15 ± 2 s −1, whereas the branching ratio experiments gave a k rad value of 3.3 ± 0.7 s −1. The large difference between these two values is due to the difference in Cr(CO) 5 − internal energy ranges probed by the two techniques. In the high internal energy regime interrogated by the two-pulse measurements (about 12,000 to 6000 cm −1), the strongly emitting C–O stretching modes are populated and contribute to fast relaxation. In contrast, the branching ratio measurements remain sensitive to internal energy changes all the way down to thermal energies, where the C–O stretches are depopulated and thus unavailable for radiative relaxation.