Substituent and solvent effects on the reaction pathway of the Beckmann rearrangement were studied. Energy surfaces of the isolated gas phase systems were mapped out using ab initio MO calculations at the MPn and QCISD(T) levels with basis sets ranging from 6-31G(d,p) to 6-311++G(2df,2p). 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 formyl substituents on C and O 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 a polarizable continuum model does affect only marginally the activation parameters with respect to the gas phase values. A combination of both quantum and;statistical mechanics was also used to probe the solvent effect. In order to investigate the active role of the solvent, ab initio calculations were carried out within a supermolecule approach. An active participation of one solvent molecule in a reacting supersystem gives rise to a genuine effect. As simple solvent molecules, H2O, H2C=O, and HCOOH were studied, the latter being a model for the Beckmann solution (HCl + acetic acid + acetic anhydride). While their involvement as a coreactant considerably reduces the barrier of the 1,2-H-shift by about 50% and hence approaches the experimental results, the effect of the bulk solvent on the reacting supersystem remains small. The 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.