Alkyl chain effect (ethyl, propyl and butyl) on the dehydrogenation mechanism of H-12-N-alkylcarbazoles has been investigated theoretically under various different conditions. Gibbs energies of activation of about 107.88 kcal mol(-1) have been determined as the least energy barriers among the studied dehydrogenation processes for dehydrogenation of H-12-N-ethylcarbazole to H-4-N-ethylcarbazole in decalin and 57.44 kcal mol(-1) for dehydrogenation of H-12-N-propylcarbazole to H-8-N-propylcarbazole under the experimental conditions. Kinetic and thermodynamic studies have shown that the route of H-4-N-alkylcarbazoles formation passes through a higher barrier than that of the H-8-N-alkylcarbazoles. Natural bond orbital (NBO) analysis showed a decrease in electron transfer between pi(C-C) and sigma(C-H)* at the center of the reaction. The electron density of the C-H bonds of the transition states was evaluated as evidence of hydrogen release via quantum theory of atoms in the molecules (QTAIM) procedure. Based on this analysis, a change in the nature of C-H bonds was confirmed from covalence to electrostatic interactions during the reaction. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.