Aqueous batteries are sustainable energy storage solutions for next-generation grid energy storage. However, their practical deployment is limited by the narrow electrochemical stability window of water, which constrains cell voltage and leads to persistent performance degradation. In this Perspective, online electrochemical mass spectrometry is highlighted as a powerful operando technique for detecting and quantifying gas evolution in aqueous batteries. The fundamental principle and historical development of the technique are briefly reviewed, followed by a systematic evaluation of recent advances in applying the technique to study common gassing events in aqueous chemistries. Perspectives on leveraging the technique for high-sensitivity, high-accuracy, and high-throughput investigations of key cell components are offered, with the goal of accelerating the development of robust and commercially viable aqueous batteries.