Room-temperature ionic liquids have been identified as nonvolatile, nonflammable compounds with a wide range of applications. However, numerous thermal studies have identified volatile decomposition products and a source for fuel, raising questions regarding the fire hazard of ionic liquids. To address these questions, the flammability properties of imidazolium-based ionic liquids have been measured using cone calorimetry and microscale combustion calorimetry. The combustion data are compared to flashpoints estimated from thermal gravimetric analysis data. The resulting flammability properties of ionic liquids are comparable to aliphatic hydrocarbon plastics (polyethylene and polyamide) and lower than high boiling organic solvents (ethyl lactate and dimethyl sulfoxide). Several structure-property relationships are observed, including alkyl chain length and anion type.