The inhibitory effects of two ionic liquids (ILs), ethyltributylphosphonium bromide ([P-4,P-4,P-4,P-2]Br) and the hydroxyl functional 1-hydroxyethyl-3-methyl imidazolium tosylate ([Hemim]Tos), on spontaneous combustion of coal were investigated and compared. The changes in various active functional groups of coal under IL inhibition were measured by in situ FTIR and analysed semi-quantitatively. Additionally, the impact of the ILs on the heat release rate of coal oxidation was examined by a micro-calorimetric technique. Subsequently, kinetic calculations were employed to elucidate the changes in the intrinsic mechanism of coal oxidation in the presence of ILs. The results revealed that [P-4,P-4,P-4,P-2]Br and [Hemim]Tos exhibited distinctive inhibition characteristics on spontaneous combustion of coal. [P-4,P-4,P-42]Br significantly reduced the associative hydroxyl group of coal by destroying the hydrogen bonds and forming stable ether structures. This kind of IL manifested significant inhibition of heat release at the slow oxidation stage of spontaneous combustion of coal. On the other hand, the reductive [Hemim]Tos afforded hydrogen ions from the functionalised O-H bonds to eliminate the reactivity of the peroxide radicals. This led to a reduction in the formation of C=O double bond intermediates in the transition and rapid oxidation stages at higher temperatures. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.