Aqueous solutions are crucial to most domains in biology and chemistry, including in energy fields such as catalysis and batteries. Water‐in‐salt electrolytes (WISEs), which extend the stability of aqueous electrolytes in rechargeable batteries, are one example. While the hype for WISEs is huge, commercial WISE‐based rechargeable batteries are still far from reality, and there remain several fundamental knowledge gaps such as those related to their long‐term reactivity and stability. Here, we propose a comprehensive approach to accelerating the study of WISE reactivity by using radiolysis to exacerbate the degradation mechanisms of concentrated LiTFSI‐based aqueous solutions. We find that the nature of the degradation species depends strongly on the molality of the electrolye, with degradation routes driven by the water or the anion at low or high molalities, respectively. The main aging products are consistent with those observed by electrochemical cycling, yet radiolysis also reveals minor degradation species, providing a unique glimpse of the long‐term (un)stability of these electrolytes.