Recently, ionic liquids were verified to be combustible instead of nonflammable; the contrary was thought to be true due their extremely low vapor pressure. Flash point is one of the most important variables used to characterize the fire and explosion hazards of liquids. Because of extremely low vapor pressure and decomposition at elevated temperatures, the reason for ionic liquids to be combustible should be different from that of traditionally defined liquids. The flash point of ionic liquids in relation to their decomposition was investigated in this study by the estimation of vapor pressure and by use of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and flash point analyzer apparatus. The ionic liquids 1-ethyl-3-methylimidazolium ethylsulfate ([Emim][EtSO₄]), 1-hexyl-3-methylimidazolium chloride ([C₆mim][Cl]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][NTf₂]), were selected as test examples. Results revealed that the flammability of ionic liquids was mainly attributed to the decomposition of the ionic liquids generating flammable substances instead of themselves vaporizing, as do traditionally defined combustible/flammable liquids. Lyon's method, applied by Fox et al. to estimate the flash point of ionic liquids from the TGA decomposition temperature, was assessed using our experimental data and the data from the published literature and resulted in substantial overestimation of the flash-point values of ionic liquids, which underestimates the fire and explosion hazards of ionic liquids. This deviation is attributed to flash-point values of ionic liquids located in the second temperature range of the TGA tracing, rather than in the third analogue, as predicted by Lyon's method.