In the pursuit of forging innovative electrosynthetic pathways and nurturing sustainable chemical processes, an in-depth comprehension of the structure and reactivity of reactive intermediates emerges as an imperative. However, the inherent challenges associated with the discernment and characterization of these transient entities arise from their meager concentrations and ephemeral nature. Herein, we present a cutting-edge floating electrolytic electrospray ionization (FE-ESI) strategy, enabling real-time monitoring and characterizing of short-lived intermediates within a floating electrolytic cell by high-resolution mass spectrometry (HRMS).Through the capture of radical cations and crucial intermediates,we acquired unambiguous evidence that sheds light on the inherent reaction process between indole radical cations and radical precursors, as well as nucleophilic reagents. This, in turn, guides the exploration of an electrochemical dearomatizative esterification of indole, culminating in the synthesis of a diverse array of 3,3-disubstituted oxindoles with notable regioselectivities.（DOI：10.1016/j.chempr.2024.02.020）