Thermal stability of five ionic liquids (ILs) was determined by thermogravimetric analysis (TGA). The ILs were selected with the common cation 1-butyl-1-methylpyrrolidinium and with the anions bis(trifluoromethylsulfonyl)imide, tetracyanoborate, trifluoromethanesulfonate, tris(pentafluoroethyl)trifluorophosphate and trifluorophosphate. Onset temperatures, tonset, were determined from dynamic experiments under air atmosphere and scanning rate of 10 °C · min-1. Additionally isothermal scans at lower temperatures than the tonset values were performed for three of the selected ILs. The results showed that the ILs with the anions bis(trifluoromethylsulfonyl)imide and trifluoromethanesulfonate present the highest thermal stability. Although the influence of the cation on this property is lower than that of the anion, these pyrrolidinium based ILs showed less thermal stabilities than those of the corresponding imidazolium based ILs with the same anions published previously. Finally, the maximum operation temperature was estimated for three possible degradations levels.