DFT calculations were performed to study the tautomeric rearrangements in the isolated, monoethanol-and diethanolsolvated, and dimeric forms of 4-methyl-1,3-dihydro-2H-1,5-benzodiazepin-2-one and 4-methyl-1,3-dihydro-2H-1,5-benzodiazepine-2-thione. Molecular geometries of the tautomers and transition state structures were optimized at the B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) levels of theory. The long-range solvent effects on the geometrical and energy parameters of tautomerization were also analyzed by use of the PCM model. The calculated relative Gibbs free energies were used to estimate the equilibrium constants. It