We performed a comparative theoretical study of the relaxation mechanisms of the excited states of uracil and 4-pyrimidinone with the CASSCF, CASPT2, and CC2 ab initio methods. The calculated vertical excitation energies agree with the experimental UV absorption maxima of the two compounds. Three low-lying conical intersections between the S0 and S1 states (one for uracil, two for 4-pyrimidinone) are established. They are accessible from the Franck–Condon region of the 1ππ* state through out-of-plane deformations related to C[double bond, length as m-dash]C (for uracil) or C[double bond, length as m-dash]N (for 4-pyrimidinone) torsions of the heterocyclic ring. These conical intersections mediate the radiationless deactivation of the compounds after excitation of the lowest 1ππ* state. The relaxation of the 1ππ* state of 4-pyrimidinoneviaC[double bond, length as m-dash]C twisting is hindered by a barrier. The relaxed scan of the C[double bond, length as m-dash]N double-bond twisting of 4-pyrimidinone indicates that the formation of the Dewar form may represent a photochemical channel in 4-pyrimidinone. This fact is detrimental for the photostability of 4-pyrimidinone, since the Dewar form is separated by a high potential-energy barrier from the canonical form of 4-pyrimidinone on the ground-state potential-energy surface, which prevents a thermal back-reaction. The investigation of the vertical excitation energies and the reaction paths shows that 4-pyrimidinone is less photostable than uracil.