Abstract The kinetics and thermodynamics of lactam/lactim tautomerization in 2-pyridone through intramolecular proton transfer have been investigated for the lowest three singlet electronic states. All the energy extrema on the three surfaces have been located and the concerted reaction paths have been traced. Our results show that planar paths are followed in the ground and the first excited ππ* singlet, which are characterized by well-defined saddle points. Activation energies are large enough to preclude direct proton transfer, but the reverse process could be operative in the ππ * state. The concerted reaction path of the nπ * state shows an energy plateau in its central part, and it was not possible to locate a sharp saddle point. This opens up the possibility of a non-planar path in this state.