The photodissociation dynamics of Mn(H)(CO)(3)(H-DAB) have been studied through wave packet propagations on CASSCF/MR-CCI potentials calculated for the electronic ground and low-lying excited states as a function of two coordinates q(a) and q(b) corresponding to the Mn-H bond homolysis and to the axial CO loss, respectively. The theoretical absorption spectrum is characterized by two bands, one intense peak centered at 476 nm (21 000 cm(-1)) and one broad band centered at 285 nm (35 000 cm(-1)). The visible band has been assigned to the low-lying metal-to-ligand charge-transfer (MLCT) states with a main contribution of the a(1)A' -> c(1)A' transition corresponding to the 3d(xz) -> pi*(DAB) excitation. The second band calculated in the UV energy domain has been assigned to the d(1)A' (3d(yz) -> 3d(xy)) and e(1)A' (sigma(Mn-H) -> pi*(DAB)) states corresponding to a metal-centered (MC) and a sigma-bond-to-ligand charge-transfer (SBLCT) state, respectively. Simulation of excited states dynamics upon visible irradiation by propagation of selected wave packets on the two-dimensional MLCT potentials coupled nonadiabatically indicates a probability of dissociation of the axial CO of 99% in 400 fs. Nonradiative transitions to the low-lying triplet states or homolysis of the metal-hydrogen bond via the dissociative (SBLCT)-S-3 state are not competitive with this ultrafast deactivation process. Simulation of the UV photochemistry points to a very low probability of dissociation in this energy region.