The analysis of the thermal stability of supported ionic liquids (ILs) is of great interest for their application in catalysis. However, thermogravimetric (TG) measurements are very time-consuming, destructive, and cannot be conducted operando. Therefore, a new radio-frequency (RF)-based method is presented that analyzes the electrical properties of supported ILs in the microwave range and can detect a possible IL mass loss caused by evaporation or decomposition. In this study, the decomposition of supported 1-butyl-3-methylimidazolium dimethylphosphate ([BMIM][DMP]) with and without palladium (as an active metal) is investigated operando during the selective hydrogenation of 1,3-butadiene. In addition to volatile decomposition products, solid products are formed, which remain on the carrier. These solid products impair the activity of the Pd catalyst. Using the RF-based method, a distinction can be made between "intact" IL and the solid decomposition products because the electrical properties of both substances differ substantially. In contrast, the destructive TG analysis only measures the mass loss by the formation of gaseous decomposition products and thus cannot distinguish between "intact" IL and the solid decomposition products of [BMIM][DMP]. In addition, a model of the thermal decomposition which depicts the measured mass losses well is presented.