Ionic liquids (ILs) are discussed in many current research papers extensively in terms of their potential use in the chemical industry, as process aids and novel materials. The long-term stability of the IL is for industrial applications as important as to know which species arise during the degradation due to thermal, mechanical, chemical or electrochemical stress. The investigation of the long-term stability of two selected ILs over several months under process-like conditions is presented with a subsequent analysis by LC-MS to identify the resulting decomposition products. Knowledge about the occurring species and their analytical quantification are basis for the selection of appropriate processes for the separation of the decomposition products and the development of recycling processes for ILs. Particularly melt crystallization processes are suitable for separating structurally similar decomposition products that typically occur in the IL degradation.