Multiconfigurational CASSCF and CASPT2 calculations were performed to investigate the enol -> keto tautomerization in the lowest singlet excited state of the 7-hydroxyquinoline center dot(NH3)(3) cluster. Two different reaction mechanisms were explored. The first one corresponds to that proposed previously by Tanner et al. (Science 2003, 302, 1736) on the basis of experimental observations and CASSCF optimizations under C-s-symmetry constraints. This mechanism comprises four consecutive steps and involves nonadiabatic transitions between the valence (1)pi pi* state and a pi sigma* Rydberg-type state, resulting in hydrogen-atom transfer. Single-point CASPT2 calculations corroborate that for C-s-symmetry pathways hydrogen-atom transfer is clearly preferred over proton transfer. The second mechanism, predicted by CASSCF optimizations without constraints, implies proton transfer along a pathway on the (1)pi pi* surface in which one or more ammonia molecules depart significantly from the molecular plane defined by the hydroxyquinoline ring. The results suggest that both mechanisms may be competitive with proton transfer being somewhat favorable over hydrogen-atom transfer.