The ionic liquids (ILs) are green solvents with numerous applications in technology and science. Among the physical properties of the ILs which have a great effect on their application, thermal stability is an important issue which has been discussed in this paper. Kinetics aspect of thermal decomposition of the ILs is considered as a scale of thermal stability. The ILs which have been studied here are composed of 1-hexyl-3-methylimidazolium cation with halide anion ([Hmim][X], (X = F, Cl, Br). Three probable paths for thermal decomposition of ILs have been proposed. The first and second paths are the SN2 mechanism in which there are two possible sites on the imidazolium ring position, methyl and hexyl chain. Third path is E2 mechanism which is initiated from hexyl chain. In the case of [Hmim][X] (X = Cl, Br), since the first path has a lower activation energy, it occurs faster than the second path, while in the case of [Hmim][F], path B has a lower barrier than path A and occurs faster. Energy analysis for three studied ILs, showed that [Hmim][Br] with Ea = 137.5 kJ mol− 1 is decomposed faster than other ILs.

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