The excited-state dynamics and photochemistry of [Re(R)(CO) 3 -(dmb)] (R Me, Et); dmb 4,4'-dimethyl-2,2'-bipyridine) in CH 2 Cl 2 have been studied by time-resolved visible absorption spectroscopy on a broad time scale ranging from approximately 400 fs to a few microseconds, with emphasis on the femtosecond and picosecond dynamics. It was found that the optically prepared Franck ± Condon 1 MLCT (singlet metal-to-ligand charge transfer) excited state of [Re(R)(CO) 3 (dmb)] undergoes femtosecond branching between two pathways (X400 fs for R Me; approximately 800 fs for R Et). For both methyl and ethyl complexes, evolution along one pathway leads to homolysis of the Re ± R bond via a 3 SBLCT (triplet s-bond-to-ligand charge transfer) excited state, from which [Re(S)(CO) 3 (dmb)] . and R . radicals are formed. The other pathway leads to an inherently unreactive 3 MLCT state. For [Re(Me)(CO) 3 -(dmb)], the 3 MLCT state lies lowest in energy and decays exclusively to the ground state with a lifetime of approximately 35 ns, thereby acting as an excitation energy trap. The reactive 3 SBLCT state is higher in energy. The quantum yield (0.4 at 293 K) of the radical formation is determined by the branching ratio between the two pathways. [Re(Et)(CO) 3 (dmb)] behaves differently: branching of the Franck ± Condon state between two pathways still occurs, but the 3 MLCT excited state lies above the dissociative 3 SBLCT state and can decay into it. This shortens the 3 MLCT lifetime to 213 ps in CH 2 Cl 2 or 83 ps in CH 3 CN. Once populated, the 3 SBLCT state evolves toward radical photoproducts [Re(S)(CO) 3 (dmb)] . and Et . . Thus, population of the 3 MLCT excited state of [Re(Et)(CO) 3 (dmb)] provides a second, delayed pathway to homolysis. Hence, the quantum yield is unity. The photochemistry and excitedstate dynamics of [Re(R)(CO) 3 (dmb)] (R Me, Et) complexes are explained in terms of the relative ordering of the Franck ± Condon, 3 MLCT, and 3 SBLCT states in the region of vertical excitation and along the Re ± R reaction coordinate. A qualitative potential energy diagram is proposed.