Abstract: In this work the thermal stability of four ionic liquids (ILs) derived from N-ethyl-N-(2-hidroxyethyl)-N,N-dimethyl ammonium cation  and N,N-di-(2-hidroxyethyl)-N-ethyl-N-methylammonium cation incorporating [NTf2] and [OTf] anions, were studied and compared with imidazolium and pyridinium ILs containing the same anions.For this proposal, a thermogravimetric analizer, TGA 7- Perkin Elmer, was used in dynamic mode from 100ºC to 800ºC at 10ºC/min under dry air atmosphere. Air atmosphere was chosen because it is more realistic to predict the stabilities in real applications such as lubrication systems or thermal fluids, which are nowadays between the main applications of ILs. From the ILs TG curves, the onset temperatures (Tonset) were determined, as well as the loss of mass at these Tonset. In all cases significant decomposition was detected at lower temperatures than Tonset, meaning that the true IL stability is lower than that provided by the dynamic TG. Isothermal studies at lower temperatures than Tonset were then carried out to estimate the long-term thermal stabilities at different temperatures.Up to now, a clear criterion on the degradation level allowed for the different ILs applications does not exist. In order to know the time that each IL takes to degrade and to obtain the maximum operation temperature allowed for every degradation level, different degradation levels (from 1% to 10%) at different temperatures were analyzed in this work.