Based on ab initio calculations using the Gaussian-4 method, we propose a new catalytic mechanism for the Beckmann rearrangement in concentrated H2SO4. Our calculations suggest that H2SO4 catalyzes the 1,2-proton-shift step via a cyclic transition structure, in which H2SO4 acts as a proton-transfer bridge. The reaction barrier for this mechanism is lower by 48.1 kJ mol(-1) than the barrier for the previously suggested catalytic mechanism, which involves a strained 3-memberd-ring transition structure. According to the previous mechanism the 1,2-proton shift has the highest activation energy, while in the revised mechanism the highest activation energy is obtained for the ensuing rearrangement/dehydration step. (C) 2016 Elsevier B.V. All rights reserved.