Potential energy (PE) surfaces of the lowest excited states of the 4(3H)-pyrimidinone/4-hydroxypyrimidine system were investigated with the aid of the CC2 and CASSCF methods of the electronic structure theory. These studies resulted in identification of a low-lying 70 state, which is dissociative with respect to the stretching of the N-H or O-H bonds in the oxo and hydroxy structures of the compound, respectively. After initial excitation to the lowest local n pi* and/or pi pi* singlet states, the system can access the PE surface of the pi sigma* state by crossing a low barrier. It was computationally demonstrated that the system should evolve on the PE surface of the repulsive pi sigma* state toward a broad seam of intersection with the PE surface of the ground state. At the intersection, the nonadiabatic transition to the ground electronic state takes place and the system can either evolve to a minimum of the initially excited tautomer or to the ground-state minimum of the other tautomer. The steps listed above provide a mechanism of photoinduced dissociation-association (PIDA) phototautomerism, experimentally observed for a number of monomeric molecules, structurally similar to 4(3H)-pyrimidinone/4-hydroxypyrimidine. This mechanism describes a new class of intramolecular phototautomeric reactions driven by a repulsive pi sigma* state.