Herein we present an innovative in situ EPR spectroscopy approach complemented with computational modeling as a methodology for assessing a nonaqueous electrolyte behavior just before its massive degradation. As a proof of concept, we use the conventional lithium electrolyte (1 M LiPF6 in EC/DMC), which is utilized in current lithium-ion batteries. Through in situ EPR, long-lived EC•- associates in amounts of 10-250 ppm were detected in a broad potential window (>2.0 V) prior to the electrolyte oxidation or reduction. The pathways of radical formation are discussed in terms of the imperfection in the electron flow across the electrolyte-electrode interface and of the strong affinity of EC to electron trapping. The radical amount could be amplified markedly (above 1000 ppm) by addition of vinylene carbonate (VC) to the electrolyte, while the added CeO2 has a moderate effect. The proposed in situ EPR methodology could be transferred to other electrolyte solutions to become a universal approach.