The thermal stability of three ionic liquids (ILs) with the same anion and different lengths of imadazolium cations was investigated using thermogravimetric analysis to consider their usefulness for their potential application as absorbents in absorption heat pumps and refrigeration cycles. First, the influence of experimental conditions such as sample mass, heating rate, and atmosphere was analyzed for one of these ILs. Afterward, dynamic and isothermal studies were performed for the three ILs. From dynamic results, onset temperature has been often selected to define the thermal stability. Nevertheless, to obtain a more precise value, long-term isothermal studies at temperatures lower than the onset one were required, showing that the thermal stability decreases slightly as the length of the cations increases. Using the Arrhenius relationship, the kinetics of isothermal decomposition was also evaluated, and an upper limit for the "working" time of these ILs as potential absorbents was estimated.