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• Thermal stability of 29 ionic liquids was assessed. • Fast (10 K/min) and slow (2 K/min) dynamic decomposition was investigated, as well as isothermal decomposition rates. • Isothermal decomposition rates for ILs under inert (N 2) versus oxidative (air) gaseous environments at different temperatures were compared. • Activation energies and vaporization enthalpies were calculated based on a zero-order kinetic model. • Decomposition mechanisms were analyzed for a selection of imidazolium, pyridinium, and tetraalkylphosphonium ILs. Ionic liquids (ILs) are being developed and studied for a variety of high-temperature applications, including as solvents for separations (where thermal regeneration is used) and as heat transfer fluids. Dynamic decomposition information has been reported for numerous ILs in the literature at various ramp rates. Although this information is useful for assessing how different characteristics, such as cation substituent groups or choice of anion, affect the decomposition of the ILs, dynamic decomposition temperature may overestimate the thermal stability of these compounds. Isothermal decomposition data is necessary when developing ILs in order to truly evaluate their thermal stability for high-temperature applications. However, this information is lacking for many ILs. In this study, we provide both fast (10 K/min) and slow (2 K/min) dynamic decomposition data, as well as isothermal decomposition data at five temperatures from 513.15 to 593.15 K for a selection of seventeen imidazolium, pyridinium, and tetraalkylammonium ILs and from 373.15 to 473.15 K for eight tetraalkylphosphonium ILs. There is significant mass loss after 16 h for most ILs even at 100 K below the decomposition temperature determined from dynamic measurements. Most ILs with the bis(trifluoromethylsulfonyl)imide anion ([Tf 2 N]−) show zero order mass loss, indicating that the mass loss is actually by evaporation rather than decomposition. Aminopyridinium ILs with [Tf 2 N]− are particularly thermally stable. We also compare isothermal decomposition rates for ILs under inert (N 2) versus oxidative (air) gaseous environments. The mass loss activation energies are lower in air, suggesting more decomposition (vs. evaporation) in air. Mass spectrometry confirms that nucleophilic substitution is the main decomposition mechanism for most ILs in N 2 , but β-elimination also occurs for the tetra-alkylphosphonium ILs. [ABSTRACT FROM AUTHOR]