A mechanistic investigation of both substituent and solvent effects in the Beckmann rearrangement is undertaken. The detailed reaction mechanism has been probed by ab initio MO calculations, both in gas phase and in solution. In the simplest gas phase system, the most favored path is as follows: protonation of formaldehyde oxime  → N-protonated species → O- protonated species → fragmentation products, in which the 1,2-H-shift connecting both protonated isomers is rate-determining. While both methyl and cyclic alkyl substituents on carbon of the oxime have only a small effect on the rate-controlling energy barrier, they significantly modify the barrier to fragmentation. The bulk solvent effect which is treated by reaction field calculations, indicates that the non-specific interaction of the solvent exerts only a small effect on both the energetic and geometrical parameters of the considered reaction path. In order to investigate the active role of the solvent, ab initio calculations were carried out within a supermolecule approach. An active partieipation of just one solvent molecule in a reacting supersystem already gives rise to a genuine effect. A series of solvent molecules including H₂O, H₂C=O and HCOOH were considered, the latter being a model for the Beckmann solution (HCl + acetic acid + acetic anhydride). Their involvement as co-reactants considerably reduces the barrier of the rate-controlling 1,2-H-shift by about 50%, and hence approaches the experimental results. Furthermore, a similar idea was also given of the reaction mechanism in concentrated sulfuric acid or in oleum solution (H₂SO₄ + S₂O₇). While a specific interaction between solvent molecules and substrates seems to be the dominant force in reducing substantially the energy barrier to 1,2-H-shift, the formation of the sulfate ester, H₂C=N-O-SO₃H, appears to play a negligible role in affecting marginally this energy barrier. Overall, calculated results suggest that the Beckmann rearrangement represents a strong case of active solvent participation, which consists in assisting the rate-determining 1,2-H-shift by catching the migrating hydrogen of the substrate and putting it back at the other end; the migration is thereby considerably accelerated.