The potential energy surfaces corresponding to the Beckmann rearrangement of protonated formaldehyde oxime and formaldehyde hydrazone in the gas phase have been explored using ab initio molecular orbital calculations. Geometries of stationary points were optimized at the HF and MP2/6–31G(d,p) level while relative energies were estimated at the MP4SDTQ/6–311++ G(d,p) level and corrected for zero-point energies. Both rearrangements are calculated to be concerted with a single transition structure, connecting the protonated species and the product complex. In the transition structure, the 1,2-hydrogen shift from C to N is accompanied by a lengthening of the N–O (in oxime) or N–N (in hydrazone) distance, and the migrating hydrogen is in trans relative to the leaving group across the CN bond. The transition structure in the hydrazone case is much closer to the final products than that in the oxime. This is also manifested by the energy barriers that amount to 44 and 189 kJ mol^–1 for oxime and hydrazone, respectively. These results suggest that the Beckmann rearrangement of oxonium ion is a facile process, while that of hydrazonium ion is reachable under thermal conditions. The experimentally observed loss of HCN from hydrazonium ion can be better rationalized in terms of a Beckmann rearrangement than a 1,2-cis-elimination as recently proposed. The proton affinities at different sites in oxime and hydrazone are also evaluated.