Isothermal and dynamic methodologies were used to study the thermal degradation of three [NTf2] anion-based ionic liquids (ILs): [C12MIM] [NTf2], [N-4441] [NTf2] and [N-8881] [NTf2]. Although it is well known that thermal stability is clearly dominated by the coordination ability of the anion, results showed that the cation exerted a significant effect, this being greater in pyrolytic conditions than in oxidising ones. The selected atmosphere also affected the thermal degradation even though the stability trend remained practically constant. In addition, the imidazolium-based IL was the one with the greatest thermal stability, with the chain length of alkyl ammonium cations also affecting their thermal stability. Longer alkyl chains reduce the intermolecular electrostatic interaction and consequently the thermal stability. As the dynamic methodology usually overestimates the long-term thermal stability, an isoconversional methodology is an excellent way for predicting the stability of these ILs in order to be used as base oil or additive in lubricants formulation. Finally, the effectiveness of the model-free methodology can be determined by comparing the results with those obtained in isothermal conditions, with both techniques in good agreement, especially for [N-8881] [NTf2].