In this study, the thermal decomposition mechanism of the amino acid ionic liquids (AAILs) including 1-ethyl-3-methylimidazolium ([Emim]) as the cation and Glycinate, Alaninate, Valinate, and Leucinate as the anions was investigated from the molecular approach. Different mechanisms of SN2, Hofmann elimination and alkyl rearrangement were proposed. These mechanisms were analyzed based on the steric hindrance effect of the alky groups and the nucleophilicity of the carboxylate and amine groups of the amino acid anions. SN2 and alkyl rearrangement mechanisms are the preferred routes of the reaction from the kinetic and thermodynamic viewpoints. Among all AAILs, [Emim][Ala] has the lowest thermal stability. To have an insight into the stability of the AAILs, topological properties and quantum chemistry reactivity indices (QCRI) were investigated and discussed. On the basis of QCRI analysis, low stability of [Emim][Ala] can be related to the lower electronic chemical hardness and greater positive character of the electrophilicity index in comparison with other AAILs. Moreover, the nature and energy of interactions at the transition states of decomposition reactions were evaluated by using the natural bond orbital method and atoms in molecules properties.

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