The potential energy (PE) surfaces of the electronic ground and the lowest excited states relevant to photophysics of interconversion of the 2-hydroxypyridine/2(1H)-pyridone system are characterized by ab initio calculations. The energy calculations at the optimized geometries are performed with the aid of the second-order perturbation theory, employing the complete-active-space self-consistent-field (CASSCF) wave function as the reference (CASPT2). Results confirm the earlier hypothesis based on the CASSCF calculations (Chem. Phys. Lett. 1993; 211, 293) that the photoinduced dissociation-association (PIDA) mechanism is probably responsible for excited-state tautomerization observed in this system. Absorption of the second photon in the excited electronic state is suggested to provide the "driving" force for the PIDA mechanism.