The imidazolium cation degrades in accordance with a three-step degradation mechanism under alkaline conditions. This mechanism usually includes a nucleophilic reaction, a ring-opening reaction, and a rearrangement reaction. In this study, we have further ascertained the degradation mechanism of a alpha-C methyl substituted imidazolium cation (trimethylimidazolium, TMIM), and of three alpha-C unsubstituted imidazolium cations (dimethylimidazolium, DMIM, 1 - benzyl-3 - methyl imidazole, BeMIM, and 1-butyl-3 - methylimidazolium, BMIM) using density functional theory (DFT). For these four Imidazolium cations, the activation energies of the nucleophilic reaction and the ring-opening reaction were calculated, along with the lowest unoccupied molecular orbital (LUMO) energy. The results obtained revealed that under alkaline conditions, the alpha-C methyl substituted imidazolium cation (TMIM) was more stable than the alpha-C unsubstituted imidazolium cations (DMIM, BeMIM, and BMIM) due to the hyper-conjugation between the methyl group at the alpha-C and the imidazole ring, and due to the stearic effect of the methyl group. The activation energies for the nucleophilic reaction and the ring-opening reaction for TMIM were 57.1 and 107.6 kj/mol respectively, and its LUMO energy was -1.013 eV. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.