Abstract The tautomerism of 20 forms of diazinones was studied by the DFT and MP2 methods combined with the aug-cc-pVDZ basis set, and, for a few structures, at the CCSD/aug-cc-pVDZ level. The energy barriers were estimated for the most probable keto–enol and amine–imine equilibria and for all OH rotations. The stabilization of diazinones primarily depends on vicinity of the N-atoms and position of the external OH/ O group. The intramolecular interaction between heteroatomic groups additionally stabilized the lactam NH C O and enol N OH systems. Six diazinones is non-Kekulean and destabilized by over 24 kcal/mol. When DFT and MP2 calculations yielded vague results, the CCSD/aug-cc-pVDZ was used. Thus it was arbitrated that 3H-pirimidin-4-one is more stable than pirimidin-4-ol by as little as ca. 0.05 kcal/mol in agreement with experimental references; 1H-pirimidin-2-one is more stable than pirimidin-2-ol and 1H-pyrazin-2-one is more stable than pyrazin-2-ol by ca. 4 kcal/mol and ca. 2.4 kcal/mol, respectively. The barriers in the NH/NH, NH/OH tautomeric equilibria were predicted to exceed 45 and 30 kcal/mol, respectively, and the barriers for OH rotation to not exceed 9 kcal/mol. The keto–enol stabilization energy differences were shown to decrease monotonically with distance between the interaction centers and the second N-atom. Because, the localized C O bond enforces double bond localization, the HOMA aromaticity of keto forms is significantly smaller than that of the hydroxy tautomers. Our previous finding that the non-Kekulean tautomers exhibit aromaticity higher than the corresponding Kekulean forms with exactly one match was again well documented.